As you probably know, the fallback mode will be dropped with GNOME 3.8 and instead, users will be able to use a set of GNOME Shell extensions that provide a GNOME2-like layout. Recently, these changes have landed in the GNOME Testing PPA for Ubuntu 12.10 and 13.04. This isn’t the old fallback session (which used to be called ‘Classic GNOME Session’ in Ubuntu), but a new session in which you’re still using GNOME Shell, but with some extensions enabled by default that try to provide a GNOME2-like layout. The first thing you’ll notice when using the new GNOME Classic session is that it comes with a light theme, as opposed to the standard GNOME Shell session which uses a dark theme by default.