He's actually a bit off; you need both.gnome-session gives the regular GNOME session as a choice (which gives you access to GNOME Shell). gnome-shell gives you GNOME Shell itself. I don't know if they fixed the problem where they collided by default, but if not, you can fix that by using this PPA:
http://www.ubuntuupdates.org/ppa/gnome_3
