Ubuntu devs discuss the change from GNOME Shell to Unity in Natty 11.04 (video)
Still, if you're interested in hearing the reasoning behind the change to Unity, you should watch community manager Jono Bacon and Desktop Experience Team manager David Barth discuss it (video after the break). You probably won't appreciate the depth of the conversation unless you're an avid Ubuntu user or hacker, though. Basically, Unity is replacing Shell because Canonical wants more control over user experience. GNOME is moving in one direction, and Ubuntu is moving in another -- and Unity bridges that gap.
What's important, though, is that Ubuntu will still be a GNOME-based operating system. Unity is an alternative to Shell, much in the same way as MeeGo; it isn't a replacement. Ultimately, Unity simply gives Ubuntu users another option; an option that Canonical thinks is better for desktop and notebook users.