Ubuntu 11.04 doesn't come with GNOME 3 / GNOME Shell (in fact it's not even available in the repositories but only in a PPA) and instead it comes with an interface designed especially for Ubuntu called Unity (based on GNOME 2.x). Unity was already default in Ubuntu 10.10 netbook edition but besides being default for the desktop edition (the netbook edition doesn't exist anymore starting with 11.04), it was also completely re-written and now uses Compiz.