I've the same problem with this configuration :
* gnome-shell 3.6.1-0ubuntu1 * mutter 3.6.1-0ubuntu1 * NVIDIA Corporation GT216 [GeForce GT 220] * Drivers 304.51.really.304.43-0ubuntu1
I have not tested the beta drivers, but the problem disappeared when I replaced my window manager by metacity.
I've the same problem with this configuration :
* gnome-shell 3.6.1-0ubuntu1 really. 304.43- 0ubuntu1
* mutter 3.6.1-0ubuntu1
* NVIDIA Corporation GT216 [GeForce GT 220]
* Drivers 304.51.
I have not tested the beta drivers, but the problem disappeared when I replaced my window manager by metacity.