The first thing I noticed when booting into Debian Wheezy was that Gnome 3 went into fallback mode. I guessed this might be because my video driver would require the firmware-linux-nonfree package to work. The procedure in Squeeze is pretty simple: just enable the non-free repository and install the package. But in wheezy it was a little bit more tricky. It only seems to be possible to add repositories by manually editing the appropriate file. I found this post on the Debian Forum, and added the entries with the exception of proposed as advised. I tried the install but the package still wasn't found. Ah, yes: I had to rebuild the package database. (Instructions for doing that and installing I found here.) I could then install the firmware.
My suspicions were confirmed: installing the firmware gave me the Gnome 3 shell!