If you really want Gnome 2, your best bet is to install the stable version of Debian. Debian Squeeze. Or stick with the LTS (Long Term Support) version of Ubuntu, 10.04. Both will be supported for several years with security updates, etc.
Ubuntu, after all, is based on Debian, so perhaps its a good time to switch to the real deal. On the other hand, Ubuntu 10.04 LTS will have more out of the box features already configured. Linux Mint 11 (Katya) is also an option, although I don't know for how long it will be supported (you could look it up).
The fallback mode in Gnome 3 superfically appears to be like Gnome 2, but lacks much of the functionality of Gnome 2. Why take a step "forward," which is really only a step backward?
Those are pretty much your options. The Gnome 2 interface has for all intents and purposes been abandoned. Unless someone forks Gnome 2, someday you'll be stuck with the backwards and shortsighted "advances" (confusing eye candy for usability) of Unity or Gnome Shell. Or as others suggested you can switch to XFCE, which is a nice, more light weight system, but not really as full featured or comparable to Gnome 2.
(By the way, there is a fork of Gnome 2 available in the AUR depository of Arch Linux, called the Mate Desktop; if you want to switch to Arch. But I'm not sure everything works properly. And there is the Fedora based Project Blue Bubble, trying to keep Gnome 2 alive, but that also I think is limited in many ways.)
Edit: It looks like the next version of Linux Mint, 12 (Lisa), coming in November, will support Gnome 2. So this is pretty good news. It would certainly offer the most up-to-date system you could have with Gnome 2. In addition, they are working with the developer of the Mate Desktop fork of Gnome 2, to keep it alive. So that's very good news, since he was working on it alone.