Interesting. When I tried gdm + gnome-shell on Ubuntu 12.10, I had two problems: one, the shell was using llvmpipe instead of a proper accelerated 3D (bad permissions on /dev/dri/card0 that I never tracked down due to issue two), and two, screen locking was badly broken and did not allow me to enter my password to unlock (and also did not let me use Ctrl+Alt+F1 or Alt+SysRq+K to recover).
I switched to lightdm and gnome-shell works now. (It didn't at first -- I would be kicked bad into lightdm as soon as I logged in, with no error information in ~/.xsession-errors. But then I logged in into Unity, logged out, and then could login into gnome-shell as well. All very mysterious.)