Was browsing through, and just throught that I'd mention, if you haven't checked it out already, try out Cairo-dock. It has a lot of useful and interesting features, including something like the icon-ring-set that you have in your mockup. Between Cairo-dock and Awn, I recommend AWN.
Its quite nice and looks useful - although, as some people before me have said, similarities with Apple's Mac OS X are undeniable; and in my opinion, thats because OS X has a very useful UI design. But, yes, there is a but, I believe that this kind of suggestions should be addressed to the GNOME team, not to Ubuntu.
The dock in the middle is patent infringement imho or regardless too similar to OSX's dock.
I think it would have been nice if you had done something cool with the top bar too since what you are showing is easily doable the way the UI is already. I like the lower glass idea, it would be consistent if it happened on the top bar.
You are going to see your realtime workspace previews real soon in ubuntu as i think that is one of the goals of gutsy (or it was a feisty goal that was deferred at least), also I think shuttleworth has asked for it.
Good work. If you produced this desktop, I would likely use it (esp the osx dock clone, oh noes!).
Linux and its various distributions and programs have infringed so many patents that no one cares anymore. Without some little patent infringements we would not have the OSs we have come to know and love.