I had Ubuntu 11.04 installed running gnome 2. I upgraded to 11.10 and everything went very smoothly. I installed GNOME Shell via:
sudo aptitude install gnome-shell
Logging into the Gnome session from Lightdm only launches Unity. I tried reinstalling gnome-shell and reconfiguring the gnome-session package to get it to rebuild the Lightdm .desktop files to no avail.