This is a short one
By enabling the Gutsy Proposed Updates, you will see bits and pieces of Gnome 2.20.1 starting to come down.
Just, a warning. These updates are called Proposed for a reason. They are mainly for testing purposes and they can sometimes cause breakage. If you enable this repository you are doing so at your own risk. They will eventually come through the regular updates. But it’s also a good way to help with testing new packages
Edit: Gnome 2.20.1 is now officially in Ubuntu Gutsy