Linux Poison posted a guide about installing GNOME 3 on Ubuntu 11.04

Unity is a shell interface for the GNOME desktop environment developed by Canonical for its Ubuntu operating system. Unity debuted in the netbook edition of Ubuntu 10.10. It is designed to make more efficient use of space given the limited screen size of netbooks, including, for example, a vertical application switcher called the launcher. Unlike GNOME, KDE Software Compilation, or Xfce, Unity is not a collection of applications but designed to be used with existing GTK+ programs.

  How to Install Gnome 3 on Ubuntu 11.04 (Natty Narwhal)