Well, as Clem has shared on the blog it isn't a good idea. The Gnome 3 developers have shared that fallback mode is transitory mode, and will be dropped from Gnome 3 much earlier than expected once they have done the work to have Gnome 3 Shell run with less powerful graphics cards (which is the reason there is a fallback mode currently, as not all graphics cards can run Gnome 3 Shell). Also, Gnome 3 fallback is not
Gnome 2. At lot of compatibility problems are there, and it takes a lot of work for things like Gnome 2 panel applets work with Gnome 3 fallback.
You may want to listen to the recent mintCast episode, where this is discussed in detail: http://www.mintcast.org/2011/10/episode-87-in-which-we-announce-our-new-schedule/
Basically, Clem is looking at using Gnome 3.2 Shell as a base and extend that to include the Linux Mint menu in some way or form that makes it minty
But there should also be a Gnome 2 / Mate based Linux Mint 12. With Mate (a recent fork of Gnome 2) it is even possible to have Gnome 3.2 and Mate in the same base install, so only one Linux Mint 12 version would be needed and you could have both Gnome 3.2 Shell and Mate for Gnome 2 as a session to choose at the login screen.