I'm using Ubuntu 12.04.1 with Gnome 3, and it just updated the kernel from 3.2.0-31 to 3.2.0-32. Now, Gnome reverts to classic mode. I can confirm that it is the kernel, because if I select 3.2.0-31 from grub, Gnome 3 works properly again.
I have a Radeon HD 7850, and I have the latest, proprietary graphics drivers installed. I haven't gone through the pain of reinstalling them yet, because I'm hoping that there's an easier way to resolve this.