Right after using Xgl & Compiz with openSuSE 10.3 on Thukydides, a Samsung P35 XVM 1600 II with an ATi Mobility Radeon 9700, I experienced a wonderful winter: every time I ended a session and wanted to log in again, my computer froze - without the ability to switch even to a console. I tried different things in the settings, reinstalled the fglrx driver and so on, but it didn't help. Finally, after some research on google I found the following tip working for me (got it from here if you're interested in the source):
Look for the file kdmrc on your machine - on mine it is localized in /opt/kde3/share/config/kdm/. As Superuser look for the entry TerminateServer=false, change it to TerminateServer=true and reboot your machine. Trying it after an update failed, because the entry was positioned wrongly - obviously it must be at the end of the section [xcore-*. ...].
At least I don't have any problems with logging out and logging in any more - happy summertime.