Ubuntu 11.04 'Natty Narwhal' will have the new Unity shell as default desktop environment, as announced by Mark Shuttleworth in his keynote at Ubuntu Developer Summit. Unity will be ported to compiz from mutter and focus will be on improved hardware compatibly, performance and quality.
Excerpt from a post about it by Jono Bacon:
I think this is a fantastic opportunity for Free Software, and this is going to be a busy cycle. We have a lot of work to do, and we know that quality is a firm focus for this release, and we have identified a solid set of issues we need to focus on and resolve, but I know the final product will be something that we will all be proud of.