Gnome 3 is released, but it is currently made compatible with Ubuntu 11.04 (Natty). The Gnome 3 PPA is still experimental and is not compatible with Unity, only choose one environment to use.
To install Gnome 2 on Ubuntu 11.04, launch the terminal and issue these commands:
sudo add-apt-repository ppa:gnome3-team/gnome3
sudo apt-get update
sudo apt-get dist-upgrade
sudo apt-get install gnome-shell