I mentioned earlier that GNOME 2.30 includes a preview of the upcoming GNOME Shell. To get this working under Fedora 13beta is very simple: you just select the GNOME Shell package via the standard "Add/Remove Software".
Since it's intended as a preview, not as a standard desktop replacement, you need to run this manually from GNOME (via a terminal window, for example.) I've experimented with it, and I like what I have seen, so far.
The GNOME project has screenshots, and a few videos, showing the new GNOME Shell in action. The version that I have is pretty close to these; the screenshots and videos are up to a year old now.
The GNOME Shell will be part of the standard desktop when GNOME 3 is eventually released, currently scheduled for September 2010.