I am getting the same error message (in syslog).  For me the symptoms are that my mouse and keyboard are still sending input to the X server, but I cannot seem to interact with any applications (ie click on buttons, menus etc).

I have to CTRL-ALT-BACKSPACE to get out of X.  This has generally yielded a working x server but earlier today it did not; I had to reboot.

This has started since I upgraded to 12.04 and/or replaced my 1280x1024 monitor with a 1920x1080 monitor.

This is a serious problem for me as it happens about once a day and my applications are generally grievously interrupted by the recovery process.

I can post logs etc if required.  Here is a fragment of syslog from just before the last occurrence to the reboot.  I tried an X kill and fresh loging before the reboot whicih I imagine explains the thread priority setting happening twice.

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May 17 14:22:22 madrid AptDaemon: INFO: Quitting due to inactivity
May 17 14:22:22 madrid AptDaemon: INFO: Quitting was requested
May 17 14:22:29 madrid rtkit-daemon[1492]: Successfully made thread 3917 of process 3917 (n/a) owned by '1000' high priority at nice level -11.
May 17 14:22:29 madrid rtkit-daemon[1492]: Supervising 4 threads of 2 processes of 2 users.
May 17 14:22:29 madrid pulseaudio[3917]: [pulseaudio] pid.c: Stale PID file, overwriting.
May 17 14:22:29 madrid rtkit-daemon[1492]: Successfully made thread 3918 of process 3917 (n/a) owned by '1000' RT at priority 5.
May 17 14:22:29 madrid rtkit-daemon[1492]: Supervising 5 threads of 2 processes of 2 users.
May 17 14:22:29 madrid rtkit-daemon[1492]: Successfully made thread 3919 of process 3917 (n/a) owned by '1000' RT at priority 5.
May 17 14:22:29 madrid rtkit-daemon[1492]: Supervising 6 threads of 2 processes of 2 users.
May 17 14:22:47 madrid goa[3951]: goa-daemon version 3.4.0 starting [main.c:112, main()]
May 17 14:23:33 madrid gnome-session[3799]: Gdk-WARNING: gnome-session: Fatal IO error 11 (Resource temporarily unavailable) on X server :0.#012
May 17 14:23:34 madrid acpid: client 3623[0:0] has disconnected
May 17 14:23:34 madrid acpid: client connected from 4144[0:0]
May 17 14:23:34 madrid acpid: 1 client rule loaded
May 17 14:23:35 madrid rtkit-daemon[1492]: Successfully made thread 4267 of process 4267 (n/a) owned by '104' high priority at nice level -11.
May 17 14:23:35 madrid rtkit-daemon[1492]: Supervising 1 threads of 1 processes of 1 users.
May 17 14:23:35 madrid rtkit-daemon[1492]: Successfully made thread 4271 of process 4267 (n/a) owned by '104' RT at priority 5.
May 17 14:23:35 madrid rtkit-daemon[1492]: Supervising 2 threads of 1 processes of 1 users.
May 17 14:23:35 madrid rtkit-daemon[1492]: Successfully made thread 4272 of process 4267 (n/a) owned by '104' RT at priority 5.
May 17 14:23:35 madrid rtkit-daemon[1492]: Supervising 3 threads of 1 processes of 1 users.
May 17 14:23:35 madrid rtkit-daemon[1492]: Successfully made thread 4275 of process 4275 (n/a) owned by '104' high priority at nice level -11.
May 17 14:23:35 madrid rtkit-daemon[1492]: Supervising 4 threads of 2 processes of 1 users.
May 17 14:23:35 madrid pulseaudio[4275]: [pulseaudio] pid.c: Daemon already running.
May 17 14:23:56 madrid kernel: Kernel logging (proc) stopped.
May 17 14:23:56 madrid rsyslogd: [origin software="rsyslogd" swVersion="5.8.6" x-pid="732" x-info="http://www.rsyslog.com"] exiting on signal 15.
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