OK, it's happened again this morning. Not after a printjob. 
The cups server is not running, and this machine is a client
to a CUPS server on another host.

The strace as in other comments gave (and it wouldn't
ctl-c, I had to kill -9 the strace):

Process 3843 attached with 9 threads - interrupt to quit
[pid 28995] gettimeofday( <unfinished ...>
[pid  3836] futex(0x80c4620, FUTEX_WAIT, 2, NULL <unfinished ...>
[pid  3837] futex(0x80c4620, FUTEX_WAIT, 2, NULL <unfinished ...>
[pid  3838] futex(0x80c4620, FUTEX_WAIT, 2, NULL <unfinished ...>
[pid  3839] futex(0x80c4620, FUTEX_WAIT, 2, NULL <unfinished ...>
[pid  3840] futex(0x80c4620, FUTEX_WAIT, 2, NULL <unfinished ...>
[pid  3841] futex(0x80c4620, FUTEX_WAIT, 2, NULL <unfinished ...>
[pid  3842] futex(0x80c4620, FUTEX_WAIT, 2, NULL <unfinished ...>
[pid 28995] <... gettimeofday resumed> {1156661704, 205634}, NULL) = 0
[pid 28995] ioctl(3, FIONREAD, [0])     = 0
[pid 28995] gettimeofday({1156661704, 206090}, NULL) = 0
[pid 28995] poll( <unfinished ...>
[pid  3843] time(NULL)                  = 1156661704
[pid  3843] recv(16, "", 583, 0)        = 0
[pid  3843] time(NULL)                  = 1156661704

with the last two lines repeated ad infinitum. That's the recv from File Descritpor 16, right? I don't know if this helps:

$lsof -p 28995
COMMAND     PID USER   FD   TYPE     DEVICE    SIZE     NODE NAME
gnome-cup 28995  jan  cwd    DIR       0,22   12288  8683893 /var/autofs/misc/home/jan (homeserver.aims.ac.za.:/home)
gnome-cup 28995  jan  rtd    DIR        3,2    4096        2 /
gnome-cup 28995  jan  txt    REG        3,2   22744   603035 /usr/bin/gnome-cups-icon
gnome-cup 28995  jan  mem    REG        0,0                0 [heap] (stat: No such file or directory)
gnome-cup 28995  jan  DEL    REG        0,8         28901402 /SYSV00000000
gnome-cup 28995  jan  mem    REG        3,2   72200   606506 /usr/lib/gtk-2.0/2.4.0/engines/libubuntulooks.so
gnome-cup 28995  jan  mem    REG        3,2  208464   652292 /usr/lib/locale/en_ZA.utf8/LC_CTYPE
gnome-cup 28995  jan  mem    REG        3,2  880086   652291 /usr/lib/locale/en_ZA.utf8/LC_COLLATE
gnome-cup 28995  jan  mem    REG        3,2   37432   475548 /lib/tls/i686/cmov/libnss_files-2.3.6.so
gnome-cup 28995  jan  mem    REG        3,2   33616   475552 /lib/tls/i686/cmov/libnss_nis-2.3.6.so
gnome-cup 28995  jan  mem    REG        3,2   32348   475544 /lib/tls/i686/cmov/libnss_compat-2.3.6.so
gnome-cup 28995  jan  mem    REG        3,2   80876   604729 /usr/lib/libsasl2.so.2.0.19
gnome-cup 28995  jan  mem    REG        3,2   44884   604574 /usr/lib/liblber.so.2.0.130
gnome-cup 28995  jan  mem    REG        3,2  213792   604580 /usr/lib/libldap_r.so.2.0.130
gnome-cup 28995  jan  mem    REG        3,2    5524   472403 /lib/libcom_err.so.2.1
gnome-cup 28995  jan  mem    REG        3,2   14164   603384 /usr/lib/libkrb5support.so.0.0
gnome-cup 28995  jan  mem    REG        3,2  142980   602792 /usr/lib/libk5crypto.so.3.0
gnome-cup 28995  jan  mem    REG        3,2  496092   603102 /usr/lib/libkrb5.so.3.2
gnome-cup 28995  jan  mem    REG        3,2  109672   602790 /usr/lib/libgssapi_krb5.so.2.2
gnome-cup 28995  jan  mem    REG        3,2   77176   475542 /lib/tls/i686/cmov/libnsl-2.3.6.so
gnome-cup 28995  jan  mem    REG        3,2  225236   604228 /usr/lib/libdbus-1.so.2.0.0
gnome-cup 28995  jan  mem    REG        3,2  737088   604160 /usr/lib/libasound.so.2.0.0
gnome-cup 28995  jan  mem    REG        3,2   38524   472421 /lib/libgcc_s.so.1
gnome-cup 28995  jan  mem    REG        3,2  849556   604771 /usr/lib/libstdc++.so.6.0.7
gnome-cup 28995  jan  mem    REG        3,2 1687716   604485 /usr/lib/libsmbclient.so.0.1
gnome-cup 28995  jan  mem    REG        3,2   10292   604438 /usr/lib/libgpg-error.so.0.1.4
gnome-cup 28995  jan  mem    REG        3,2  310348   604318 /usr/lib/libgcrypt.so.11.2.1
gnome-cup 28995  jan  mem    REG        3,2   63656   604776 /usr/lib/libtasn1.so.2.0.17
gnome-cup 28995  jan  mem    REG        3,2   17416   604083 /usr/lib/libORBitCosNaming-2.so.0.1.0
gnome-cup 28995  jan  mem    REG        3,2  142304   604691 /usr/lib/libpng12.so.0.1.2.8
gnome-cup 28995  jan  mem    REG        3,2    7052   604097 /usr/lib/libXau.so.6.0.0
gnome-cup 28995  jan  mem    REG        3,2  125168   604281 /usr/lib/libexpat.so.1.0.0
gnome-cup 28995  jan  mem    REG        3,2    7648   475565 /lib/tls/i686/cmov/libutil-2.3.6.so
gnome-cup 28995  jan  mem    REG        3,2   68804   475559 /lib/tls/i686/cmov/libresolv-2.3.6.so
gnome-cup 28995  jan  mem    REG        3,2   57168   604177 /usr/lib/libavahi-client.so.3.1.2
gnome-cup 28995  jan  mem    REG        3,2   38892   604179 /usr/lib/libavahi-common.so.3.4.0
gnome-cup 28995  jan  mem    REG        3,2    8800   604181 /usr/lib/libavahi-glib.so.1.0.0
gnome-cup 28995  jan  mem    REG        3,2  132104   604175 /usr/lib/libaudiofile.so.0.0.2
gnome-cup 28995  jan  mem    REG        3,2   37400   604268 /usr/lib/libesd.so.0.2.36
gnome-cup 28995  jan  mem    REG        3,2  430292   602681 /usr/lib/libfreetype.so.6.3.8
gnome-cup 28995  jan  mem    REG        3,2  123372   604560 /usr/lib/libjpeg.so.62.0.0
gnome-cup 28995  jan  mem    REG        3,2 1232784   475531 /lib/tls/i686/cmov/libc-2.3.6.so
gnome-cup 28995  jan  mem    REG        3,2  176644   604418 /usr/lib/libgnomecupsui-1.0.so.1.0.0
gnome-cup 28995  jan  mem    REG        3,2  535424   603443 /usr/lib/libglib-2.0.so.0.1000.3
gnome-cup 28995  jan  mem    REG        3,2   39124   604400 /usr/lib/libgnome-keyring.so.0.0.1
gnome-cup 28995  jan  mem    REG        3,2  229092   604297 /usr/lib/libgobject-2.0.so.0.1000.3
gnome-cup 28995  jan  mem    REG        3,2   18900   475535 /lib/tls/i686/cmov/libcrypt-2.3.6.so
gnome-cup 28995  jan  mem    REG        3,2   86580   475557 /lib/tls/i686/cmov/libpthread-2.3.6.so
gnome-cup 28995  jan  mem    REG        3,2  426584   604434 /usr/lib/libgnutls.so.12.3.6
gnome-cup 28995  jan  mem    REG        3,2  182496   603033 /usr/lib/libcups.so.2
gnome-cup 28995  jan  mem    REG        3,2   51552   605055 /usr/lib/libgnomecups-1.0.so.1.0.0
gnome-cup 28995  jan  mem    REG        3,2   13324   604670 /usr/lib/libgthread-2.0.so.0.1000.3
gnome-cup 28995  jan  mem    REG        3,2    8204   475537 /lib/tls/i686/cmov/libdl-2.3.6.so
gnome-cup 28995  jan  mem    REG        3,2    9768   604643 /usr/lib/libgmodule-2.0.so.0.1000.3
gnome-cup 28995  jan  mem    REG        3,2  136368   475539 /lib/tls/i686/cmov/libm-2.3.6.so
gnome-cup 28995  jan  mem    REG        3,2  327152   604079 /usr/lib/libORBit-2.so.0.1.0
gnome-cup 28995  jan  mem    REG        3,2   75844   604193 /usr/lib/libbonobo-activation.so.4.0.0
gnome-cup 28995  jan  mem    REG        3,2  352324   604191 /usr/lib/libbonobo-2.so.0.0.0
gnome-cup 28995  jan  mem    REG        3,2  938112   604091 /usr/lib/libX11.so.6.2.0
gnome-cup 28995  jan  mem    REG        3,2  286492   604199 /usr/lib/libcairo.so.2.2.4
gnome-cup 28995  jan  mem    REG        3,2  225076   603382 /usr/lib/libpango-1.0.so.0.1201.2
gnome-cup 28995  jan  mem    REG        3,2   13776   604110 /usr/lib/libXfixes.so.3.0.0
gnome-cup 28995  jan  mem    REG        3,2   32548   604102 /usr/lib/libXcursor.so.1.0.2
gnome-cup 28995  jan  mem    REG        3,2    9772   604128 /usr/lib/libXrandr.so.2.0.0
gnome-cup 28995  jan  mem    REG        3,2   26744   604116 /usr/lib/libXi.so.6.0.0
gnome-cup 28995  jan  mem    REG        3,2    6436   604118 /usr/lib/libXinerama.so.1.0.0
gnome-cup 28995  jan  mem    REG        3,2   29348   604130 /usr/lib/libXrender.so.1.3.0
gnome-cup 28995  jan  mem    REG        3,2   51908   604108 /usr/lib/libXext.so.6.4.0
gnome-cup 28995  jan  mem    REG        3,2  179640   604290 /usr/lib/libfontconfig.so.1.0.4
gnome-cup 28995  jan  mem    REG        3,2   27972   606296 /usr/lib/libpangocairo-1.0.so.0.1201.2
gnome-cup 28995  jan  mem    REG        3,2   84616   608046 /usr/lib/libgdk_pixbuf-2.0.so.0.800.20
gnome-cup 28995  jan  mem    REG        3,2   97812   604169 /usr/lib/libatk-1.0.so.0.1114.0
gnome-cup 28995  jan  mem    REG        3,2  512260   608052 /usr/lib/libgdk-x11-2.0.so.0.800.20
gnome-cup 28995  jan  mem    REG        3,2   77368   604831 /usr/lib/libz.so.1.2.3
gnome-cup 28995  jan  mem    REG        3,2 1103284   604819 /usr/lib/libxml2.so.2.6.24
gnome-cup 28995  jan  mem    REG        3,2 2950692   608053 /usr/lib/libgtk-x11-2.0.so.0.800.20
gnome-cup 28995  jan  mem    REG        3,2   93620   604382 /usr/lib/libglade-2.0.so.0.0.7
gnome-cup 28995  jan  mem    REG        3,2  195296   604316 /usr/lib/libgconf-2.so.4.1.0
gnome-cup 28995  jan  mem    REG        3,2  371236   603428 /usr/lib/libgnomevfs-2.so.0.1400.2
gnome-cup 28995  jan  mem    REG        3,2   26592   472458 /lib/libpopt.so.0.0.0
gnome-cup 28995  jan  mem    REG        3,2   81736   604396 /usr/lib/libgnome-2.so.0.1401.0
gnome-cup 28995  jan  mem    REG        3,2  147200   606288 /usr/lib/libpangoft2-1.0.so.0.1201.2
gnome-cup 28995  jan  mem    REG        3,2   81120   604158 /usr/lib/libart_lgpl_2.so.2.3.17
gnome-cup 28995  jan  mem    REG        3,2  167988   604414 /usr/lib/libgnomecanvas-2.so.0.1400.0
gnome-cup 28995  jan  mem    REG        3,2  364092   604195 /usr/lib/libbonoboui-2.so.0.0.0
gnome-cup 28995  jan  mem    REG        3,2   84724   604071 /usr/lib/libICE.so.6.3.0
gnome-cup 28995  jan  mem    REG        3,2   31328   604087 /usr/lib/libSM.so.6.0.0
gnome-cup 28995  jan  mem    REG        3,2  540660   604340 /usr/lib/libgnomeui-2.so.0.1401.0
gnome-cup 28995  jan  mem    REG        3,2   14720   603143 /usr/lib/gtk-2.0/2.4.0/loaders/libpixbufloader-png.so
gnome-cup 28995  jan  mem    REG        3,2   14744   475546 /lib/tls/i686/cmov/libnss_dns-2.3.6.so
gnome-cup 28995  jan  mem    REG        3,2    5576   605904 /usr/lib/gconv/ISO8859-1.so
gnome-cup 28995  jan  mem    REG        3,2      54   652297 /usr/lib/locale/en_ZA.utf8/LC_NUMERIC
gnome-cup 28995  jan  mem    REG        3,2    2375   652300 /usr/lib/locale/en_ZA.utf8/LC_TIME
gnome-cup 28995  jan  mem    REG        3,2     286   652295 /usr/lib/locale/en_ZA.utf8/LC_MONETARY
gnome-cup 28995  jan  mem    REG        3,2      48   667890 /usr/lib/locale/en_ZA.utf8/LC_MESSAGES/SYS_LC_MESSAGES
gnome-cup 28995  jan  mem    REG        3,2      34   652298 /usr/lib/locale/en_ZA.utf8/LC_PAPER
gnome-cup 28995  jan  mem    REG        3,2      73   652296 /usr/lib/locale/en_ZA.utf8/LC_NAME
gnome-cup 28995  jan  mem    REG        3,2     159   652290 /usr/lib/locale/en_ZA.utf8/LC_ADDRESS
gnome-cup 28995  jan  mem    REG        3,2      58   652299 /usr/lib/locale/en_ZA.utf8/LC_TELEPHONE
gnome-cup 28995  jan  mem    REG        3,2      23   652294 /usr/lib/locale/en_ZA.utf8/LC_MEASUREMENT
gnome-cup 28995  jan  mem    REG        3,2     454   652293 /usr/lib/locale/en_ZA.utf8/LC_IDENTIFICATION
gnome-cup 28995  jan  mem    REG        3,2   86404   472374 /lib/ld-2.3.6.so
gnome-cup 28995  jan    0r   CHR        1,3             5263 /dev/null
gnome-cup 28995  jan    1w  FIFO        0,6            15775 pipe
gnome-cup 28995  jan    2w  FIFO        0,6            15775 pipe
gnome-cup 28995  jan    3u  unix 0xe2769100         13205103 socket
gnome-cup 28995  jan    4r  FIFO        0,6         13205106 pipe
gnome-cup 28995  jan    5w  FIFO        0,6         13205106 pipe
gnome-cup 28995  jan    6r  FIFO        0,6         13205107 pipe
gnome-cup 28995  jan    7w  FIFO        0,6         13205107 pipe
gnome-cup 28995  jan    8r  FIFO        0,6         13205108 pipe
gnome-cup 28995  jan    9w  FIFO        0,6         13205108 pipe
gnome-cup 28995  jan   10u  unix 0xdf1eb080         13205110 socket
gnome-cup 28995  jan   11u  unix 0xe8ada580         13205112 socket
gnome-cup 28995  jan   12u  unix 0xca0e2300         13205114 /tmp/orbit-jan/linc-7143-0-4c9382b623596
gnome-cup 28995  jan   13u  unix 0xd7cceb00         13205117 /tmp/orbit-jan/linc-7143-0-4c9382b623596
gnome-cup 28995  jan   14u  unix 0xd7cce900         13205122 /tmp/orbit-jan/linc-7143-0-4c9382b623596
gnome-cup 28995  jan   15u  unix 0xe2769300         13205119 socket
gnome-cup 28995  jan   16u  IPv4   15266655              TCP dikkop.aims.ac.za:51384->heron.aims.ac.za:ipp (CLOSE_WAIT)
gnome-cup 28995  jan   17u  unix 0xc74e9180         15368854 socket

Is file descriptor 16 that second last line? The TCP connection to 
the CUPS server? Also, what is the FD=DEL line near the top
of the lsof output on /SYSV00000000? I'm not expert enough to make total sense out of all of this. Need any more output?