I installed gnome 3.6 in ubuntu from ricotz' staging and testing ppa's. My experience wasn't very stable though (sound not working occasionally, crashed of gnome-settings-daemon) so I wanted to downgrade again. Unfortunately, ppa-purge would suggest very invasive changes to resolve conflicts (removing lightdm, for example).
Here's the command that you can use yourself to downgrade packages to the latest version that is in the archive:
sudo ppa-purge ppa:ricotz/testing
(now press N if you are suggested inappropriate changes)
sudo aptitude install '?version(ricotz)'/quantal
The ?version(ricotz) expression selects all packages that have the string 'ricotz' in their version name; I'm very happy that ricotz marks all his packages like that. The suffix /quantal tells apt to select the latest version from the quantal archive, instead of the latest version it knows of (which would be, of course, the installed one).