With the great work from Robert Ancell and Sebastien Bacher, who worked on packaging the new GLib/GTK3 stack, and with the recent packaging of a few GNOME 3 applications (eog, Nautilus, the new control center, …), you can start testing what will be GNOME 3 on Ubuntu (Natty) by using this PPA.
Please note that this is a work very much in progress, which means that, apart from the usual problems of running unstable software, it’s got the unstability of new packages added, so please USE WITH CARE. I would suggest to use a virtual machine for testing this, but please test it and report any problems you might find. It seems to be running ok for me (on a virtual machine), but please don’t risk your every day desktop 😀