I'm using Ubuntun 12.10 and GNOME Shell 3.6.2. When enabling hamster and shell-extension (both freshly checked out from github) I experience a rather nasty problem.
After locking my machine the lockscreen shows up. I input my password but then the view won't change back to my desktop but the background of the lockscreen stays on top. I then have to restart gdm to access my desktop.
I'm having this exact same problem, also Ubuntu 12.10, GDM and GNOME Shell 3.6.2.
Original issue is here projecthamster/hamster-shell-extension#50. It was fixed, but sometimes I still get the screen freezed.