The Ubuntu Unity Desktop was developed by Canonical for Ubuntu Light during the Ubuntu 10.10 development cycle and was deployed with the Ubuntu Netbook Edition for the Maverick Meerkat. Mark Shuttleworth announced this morning when kicking off the Ubuntu 11.04 Developer Summit in Orlando the decision to abandon GNOME's shell in favor of their own solution. The Unity Desktop though does use Mutter for its window manager and the GNOME Shell still should be available via their package repositories.
Ubuntu 11.04 will still ship GNOME applications, but the shell is being replaced. Canonical's Jono Bacon has already blogged about this confirming Mark's statements about Unity really going into the Ubuntu desktop. Jono also had this to say, "There is going to be some questions about this decision in relation to GNOME. I want to make something crystal clear: Ubuntu is GNOME distribution, we ship the GNOME stack, we will continue to ship GNOME apps, and we optimize Ubuntu for GNOME. The only difference is that Unity is a different shell for GNOME, but we continue to support the latest GNOME Shell development work in the Ubuntu archives."