I have a Natty system where I wanted to early install GTK3 ported packages such as Evince, LightDM,...
To do so, I just temporarily enabled Oneiric repositories (without no more pinning configuration, I love my hairs) then installed
I got the lightDM login screen without problem. For some obscur reasons, I had to disable splashscreen at boot. That doesn't disturb me, I love seeing lines on my screen, it's an indication that things are going on.
But, after entering my credentials and choosing "Xubuntu Session" or "Xfce Session", I just got a framebuffer screen with a mouse pointer. Not more. I can escape from that state with "Ctrl-Backspace".
I joined the bug huntings:
Solution for me
Completely disappointed, I randomly tried to enter a "guest" session on the computer and the Xfce environment launched.
I tried then to move out all my $HOME files and directories and gave a try: It worked!
So, something in my ".config/" or any dotfile prevented Xfce to launch.
If you encouter that bug, try oyrself and tell me!