Cinnamon is GNOME3 based fork that is designed with GNOME 2.x users in mind (similar to what we see on Linux Mint 12). This shell is already available for download and if you want, you can test by downloading form here (can be very buggy, mind it!). So, what extra does it provide over Mint 12's MGSE extensions?
"Cinnamon is a Linux desktop which provides advanced innovative features and a traditional user experience. The desktop layout is similar to Gnome 2. The underlying technology is forked from Gnome Shell." says Cinnamon developer Clement Lefebvre.According to Clement, though the layout looks a bit similar to GNOME 3 with MGSE, there is more functionality to this.