Edit:
You can even use Gnome 3 in ubuntu 11.04 (But you won't be able to downgrade to Unity again or use it side-by-side with gnome 3, because gnome 3 ppa breaks unity session.
How to Install GNOME 3 on Ubuntu 11.04 - Softpedia
Just install ppa-purge
and run
ppa-purge ppa:<gnome3ppayouusedtoinstallgnome3>
e.g.
Get back to unity and classic ubuntu with gnome 2 in jiffy.ppa-purge ppa:gnome3-team/gnome3