Using gdm, xdm or kdm, whenever I log in, I am just brought right back to the login screen (it seems that the session dies immidiately). I have tried it with every session availible (gnome, kde, failsafe, anotherlevel), and each give the same result. The problem didn't used to happen; it started suddenly (I logged out, but couldn't log in afterwards). startx works fine, however.
The problem seems to have fixed itself as suddenly as it appeared.
I've seen this on occasion myself, and it appears to be due to some sort of library access problem in the PAM system, although it's not consistant as to which library causes the problem. Sorry I can't help more though...