I have installed Ubuntu 12.04 beta, and after installing the gnome-shell each time before i log in, when I select Gnome desktop, I get the gnome backfall (classic) desktop.
I also noticed that I cannot enable compiz, (I could with the 10.10 version), I dont know if it has something to do with the drivers.
I have a intel i3 processor
If you are using an optimus based laptop, going to the Ubuntu Software Center and removing the "NVidia binary X.Org driver ('current' driver)" may solve the problem.
I managed to fix this issue installing bumblebee, after the restart everything is working correctly, Shell & Compiz on Gnome Classic
sudo apt-add-repository ppa:mj-casalogic/bumblebee
sudo apt-get update && sudo apt-get install bumblebee