I'm also seeing this on a 12.04 minimal install. After logrotate runs, when syslog stops being written to, here's what my /var/log directory looks like:

-rw-r--r-- 1 root     root       3721 Oct 11 16:11 alternatives.log
drwxr-xr-x 2 root     root       4096 Aug 21 22:02 apt
-rw-r--r-- 1 root     root       6966 Oct 11 16:09 aptitude
drwxr-xr-x 4 asterisk asterisk   4096 Oct 11 16:11 asterisk
-rw-r----- 1 syslog   adm        7700 Oct 12 09:41 auth.log
-rw-r----- 1 root     adm          31 Aug 21 22:06 boot
-rw-r--r-- 1 root     root       1136 Oct 11 16:07 boot.log
-rw-rw---- 1 root     utmp          0 Aug 21 22:37 btmp
-rw-r--r-- 1 root     root          0 Aug 21 22:37 debug
-rw-r--r-- 1 root     adm       13399 Oct 11 16:07 dmesg
-rw-r--r-- 1 root     adm       13399 Oct 11 16:06 dmesg.0
-rw-r--r-- 1 root     root         28 Oct 11 16:06 dmesg.1.gz
-rw-r--r-- 1 root     root      97979 Oct 11 16:11 dpkg.log
-rw-r--r-- 1 root     root      24024 Oct 11 16:11 faillog
drwxr-xr-x 2 root     root       4096 Aug 21 22:06 fsck
-rw-r----- 1 syslog   adm       63451 Oct 11 16:08 kern.log
-rw-rw-r-- 1 root     utmp     292292 Oct 12 09:41 lastlog
-rw-r--r-- 1 root     root          0 Aug 21 22:37 mail.*
-rw-r----- 1 syslog   adm           0 Oct 11 16:05 mail.err
-rw-r----- 1 syslog   adm           0 Oct 11 16:05 mail.log
-rw-r--r-- 1 root     root          0 Aug 21 22:37 messages
drwxr-xr-x 2 root     root       4096 Oct 11 16:05 news
-rw-r--r-- 1 root     root          0 Oct 12 07:35 syslog
-rw-r--r-- 1 root     root      22423 Oct 11 16:07 syslog.1
-rw-r--r-- 1 root     root      85907 Oct 11 16:07 udev
drwxr-xr-x 2 root     root       4096 Oct 12 07:35 upstart
-rw-rw-r-- 1 root     utmp      13056 Oct 12 09:41 wtmp

... and here's what it looks like after a reboot:

-rw-r--r-- 1 root     root       3721 Oct 11 16:11 alternatives.log
drwxr-xr-x 2 root     root       4096 Aug 21 22:02 apt
-rw-r--r-- 1 root     root       6966 Oct 11 16:09 aptitude
drwxr-xr-x 4 asterisk asterisk   4096 Oct 11 16:11 asterisk
-rw-r----- 1 syslog   adm        8206 Oct 12 09:51 auth.log
-rw-r----- 1 root     adm          31 Aug 21 22:06 boot
-rw-r--r-- 1 root     root       1073 Oct 12 09:48 boot.log
-rw-rw---- 1 root     utmp          0 Aug 21 22:37 btmp
-rw-r--r-- 1 root     root          0 Aug 21 22:37 debug
-rw-r--r-- 1 root     adm       13399 Oct 12 09:48 dmesg
-rw-r--r-- 1 root     adm       13399 Oct 11 16:07 dmesg.0
-rw-r--r-- 1 root     adm        5228 Oct 11 16:06 dmesg.1.gz
-rw-r--r-- 1 root     root         28 Oct 11 16:06 dmesg.2.gz
-rw-r--r-- 1 root     root      97979 Oct 11 16:11 dpkg.log
-rw-r--r-- 1 root     root      24024 Oct 11 16:11 faillog
drwxr-xr-x 2 root     root       4096 Aug 21 22:06 fsck
-rw-r----- 1 syslog   adm       85551 Oct 12 09:48 kern.log
-rw-rw-r-- 1 root     utmp     292292 Oct 12 09:51 lastlog
-rw-r--r-- 1 root     root          0 Aug 21 22:37 mail.*
-rw-r----- 1 syslog   adm           0 Oct 11 16:05 mail.err
-rw-r----- 1 syslog   adm           0 Oct 11 16:05 mail.log
-rw-r--r-- 1 root     root          0 Aug 21 22:37 messages
drwxr-xr-x 2 root     root       4096 Oct 11 16:05 news
-rw-r--r-- 1 root     root      22260 Oct 12 09:49 syslog
-rw-r--r-- 1 root     root      22423 Oct 11 16:07 syslog.1
-rw-r--r-- 1 root     root      85907 Oct 12 09:48 udev
drwxr-xr-x 2 root     root       4096 Oct 12 09:49 upstart
-rw-rw-r-- 1 root     utmp      20736 Oct 12 09:51 wtmp

Restarting rsyslogd doesn't seem to fix this for me, I actually have to reboot the VM to get it to start writing logs again.