Unity (which now shows up as a plugin in CompizConfig Settings Manager) has finally been made default in Ubuntu (desktop) 11.04 Natty Narwhal yesterday, as you can see in the Ubuntu meta changelog. That means that if you download the latest Ubuntu 11.04 Natty Narwhal daily build and boot from it, you should get Unity without having to install / enable anything. The same changelog mentions that Screen has been removed from the desktop version.
But back to Unity: there's not much that works right now, but here is a video I've recorded with the latest Unity in Ubuntu 11.04 Natthy Narwhal (as of today):
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As you can see, there's no Dash yet (the Ubuntu logo in the top left corner just triggers the Unity launcher unhide) so you must use a terminal to launch applications, the quicklist menu items have some random text (for testing purposes for now) so Unity is not really usable yet.
But there are some things that already work: you can set the Unity launcher ("dock") to autohide or float and the desktop switcher finally works.
There is A LOT of work to be done, but let's not forget that Ubuntu 11.04 Natthy Narwhal alpha 1 is expected on December 2nd, so we're just at the beginning of the road.