I had this weird problem after a log-out in a tight situation. What happened afterwards (and after rebooting) was that I only got a blue screen that flashed every minute or so. After a lot of tests and verbose booting, I noticed that the loginwindow.app was systematically crashing. What happened was that its prefs had a wrong user in the "previous user" field. The fix was:
<key>lastUser</key>change loggedOut to your user name (I'm not exactly sure that it's loggedOut, but it's something close to that, I didn't write it down).
<string>loggedOut</string>
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020525102218715