Here, an additional test Test. A new / clean Ubuntu Lucid installation:

root@ts3:~# lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 10.04.4 LTS
Release:	10.04
Codename:	lucid


I set this limits in /etc/security/limits.conf
root hard nofile 65536
root soft nofile 65536


root@ts3:~# ulimit -a
core file size          (blocks, -c) 0
data seg size           (kbytes, -d) unlimited
scheduling priority             (-e) 20
file size               (blocks, -f) unlimited
pending signals                 (-i) 16382
max locked memory       (kbytes, -l) 64
max memory size         (kbytes, -m) unlimited
open files                      (-n) 65536
pipe size            (512 bytes, -p) 8
POSIX message queues     (bytes, -q) 819200
real-time priority              (-r) 0
stack size              (kbytes, -s) 8192
cpu time               (seconds, -t) unlimited
max user processes              (-u) unlimited
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited


root@ts3:~# dpkg -l "cups*"
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Cfg-files/Unpacked/Failed-cfg/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name           Version        Description
+++-==============-==============-============================================
ii  cups           1.4.3-1ubuntu1 Common UNIX Printing System(tm) - server
un  cups-bsd       <none>         (no description available)
ii  cups-client    1.4.3-1ubuntu1 Common UNIX Printing System(tm) - client pro
ii  cups-common    1.4.3-1ubuntu1 Common UNIX Printing System(tm) - common fil
ii  cups-driver-gu 5.2.5-0ubuntu1 printer drivers for CUPS
un  cups-pdf       <none>         (no description available)
un  cups-ppdc      <none>         (no description available)
un  cups-pt        <none>         (no description available)
un  cupsddk        <none>         (no description available)
un  cupsddk-driver <none>         (no description available)
un  cupsomatic-ppd <none>         (no description available)
un  cupsys         <none>         (no description available)
un  cupsys-bsd     <none>         (no description available)
un  cupsys-client  <none>         (no description available)
un  cupsys-common  <none>         (no description available)
un  cupsys-driver- <none>         (no description available)


CPUS changes in /etc/cups/cupsd.conf:
LogLevel debug
MaxClients 32768


CUPS 1.4.3-1ubuntu1 works fine, like expected:

root@ts3:~# grep Max /var/log/cups/error_log
I [22/Feb/2012:18:17:23 +0100] MaxClients limited to 1/3 (21845) of the file descriptor limit (65536)...


then i change the /etc/apt/sources.list from lucid to maverick and reinstall CUPS
apt-get install cups
...
The following packages will be upgraded:
cups cups-client cups-common libcups2


root@ts3:~# dpkg -l "cups*"
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Cfg-files/Unpacked/Failed-cfg/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name           Version        Description
+++-==============-==============-============================================
ii  cups           1.4.4-6ubuntu2 Common UNIX Printing System(tm) - server
un  cups-bsd       <none>         (no description available)
ii  cups-client    1.4.4-6ubuntu2 Common UNIX Printing System(tm) - client pro
ii  cups-common    1.4.4-6ubuntu2 Common UNIX Printing System(tm) - common fil
ii  cups-driver-gu 5.2.5-0ubuntu1 printer drivers for CUPS
un  cups-pdf       <none>         (no description available)
ii  cups-ppdc      1.4.4-6ubuntu2 Common UNIX Printing System(tm) - PPD manipu
un  cups-pt        <none>         (no description available)
un  cupsddk        <none>         (no description available)
un  cupsddk-driver <none>         (no description available)
un  cupsomatic-ppd <none>         (no description available)
un  cupsys         <none>         (no description available)
un  cupsys-client  <none>         (no description available)
un  cupsys-driver- <none>         (no description available)


and here the error:
I [22/Feb/2012:18:21:54 +0100] MaxClients limited to 1/3 (341) of the file descriptor limit (1024)...

The only change is CUPS upgrade from 1.4.3-1ubuntu1 to 1.4.4-6ubuntu2, configuration has not changed.

BR Pawel