If you have started using Acrobat Reader 9.1 on Linux, there is a nasty little bug that took me a while to locate. It's leaking files into /dev/shm that don't delete, and cause problems on the next startup. We were getting calls from people that when they clicked on Reader, their whole Xserver would freeze until the stuck process was removed. After running strace, I was able to see that it was attempting to use /dev/shm right before the lockup; and found the leaking files.
I placed these lines into the script that calls Reader and things are working as expected:
rm /dev/shm/sem*$USER 2> /dev/null