Writing this from an older Dell Notebook, ugpraded to 12.04 32-Bit, which works well.
My more modern Dell Notebook worked well for 1/2 day after upgrading to 12.04 64-Bit, now I'm having problems logging in:
System boots normally, (Unity) can enter my password, the login-screen reports 'Logging in....', but that's the end, won't log in even after 30 minutes - it just stays on the login screen. Same for guest login.
Console log in works well and quick. Checked Xorg.0.log.old, but didn't see an error message. The user:group for my home directory is correct. An fsck didn't find any errors.
Will need to find out how to mount an USB stick from command line, sure you'll want me to upload log files...