Staring at my monitor (not typing, working with programs or whatever) the X server suddenly seems to restart, presenting me the login window. This is what i got from dmesg:
[35481.933340] compiz.real: segfault at 00003e27 eip 08055a6d esp bf9d3f80 error 4
A restarting server is always a sign of a bug inside the server or (more likely) the video card driver. Compiz segfaulting most likely is just a symptom, not the real problem.
Please report this problem to the developers of your driver.