Ubuntu is a Gnome based OS. 11.04 is based on Gtk2 and Gnome2.
You have the choice of desktop environments. Unity or Gnome DE.
11.10 will be Gnome based using Gtk3 and Gnome3.
You will have the choice of Unity or Gnome DE.
The Gnome DE will be different under Gnome3 than what we are used to in Gnome2 just as Unity is different. Gnome-Shell, the new Gnome DE, has also been available for some time now (it was available during the 10.04-testing cycle). Unity was first available during the 10.10-testing cycle.
I have an install of Debian Sid with Gnome Shell and the 3.0.0 linux kernel. If you want to look at GS integrated into an OS go to the Gnome site and they will direct you to an Open Suse Live CD with GS as the default DE or a Fedora15 Live CD with GS as default DE. I recommend the F15, in spite of no amd64 version (last time I checked), because it has a bunch of the community developed extensions for GS in the repos.
Ubuntu is having a time getting it integrated with Ubuntu and Unity but should have it as good as Ubuntu ever does by release time.
As for how Unity works. All I can say, from personal use of it, is poorly. So far my impression of GS is that it is less bad.
I will be switching from Gnome to Xfce (Xubuntu uses it as the default DE) on my Debian OS'. My wife may be sticking to Ubuntu with Gnome Shell.