Through my sojourns through the Ask Ubuntu world, I've seen this question several times:
"How do I switch to ?"
- How do I switch to the Classic GNOME Desktop?
Switching between Kubuntu and Ubuntu without re-installation
While this one doesn't ask that exact question, it is similar, and should have been mentioned in the answer.
I think there were XFCE, LXDE and Unity 2D questions, but I can't find them so maybe not.
Anyhow, here's the problem:
The answer of all of these will include
Logout, select [desktop environment name here], and log back in.
So, is this something we want? With Unity being the default desktop period (no way to escape to normal GNOME2!) in 11.10, we're likely to get a lot more of this kind of question.