A quick update: AMD has released Catalyst 11.4 with support for X Server 1.10 so all of you who want to upgrade to Ubuntu 11.04 Natty Narwhal should now be able to use Unity.
I do not use an ATI graphics card so I can't really tell you how everything works so all I can do is give you a download link:
If you have an ATI graphics card, please let us know your experience with the latest 11.04 in the comments!
Many thanks to WebUpd8 reader _artem_ for the screenshot!