http://linuxfud.wordpress.com/2006/0...-login-screen/
If any of you have a gnome panel set to be transparent, this may be the issue.
I deleted all my gnome files via a console login, restarted, and all was fine, then added all my settings but avoided using panel transparency, as much as it looks cool.
This is a serious gnome bug, as a feature which is minor decoration can cause complete system collapse, which, at least for a noob like me, took days to resolve, chasing driver and permission issues, only to find it was a simple setting on the desktop.
best,
M