Half-left has released what's probably the first "real" theme for the new Gnome Shell (with the new automatic workspaces, overview re-layout and so on). Gnome Shell already looks pretty cool but check out Half-left's theme called "Smooth Inset":
The theme only works with the latest Gnome Shell from GIT (at least version 2.91.90) - check out or Gnome Shell Ubuntu 10.10 installation post if you want to build the latest Gnome Shell from GIT.
Unfortunately there are no built-in settings in Gnome Shell to allow changing the whole Shell theme so to install it you'll have to manually copy some files to the ~/gnome-shell/source/gnome-shell/data/ folder. Hopefully this will change soon.
I expect to see a lot more amazing themes for Gnome Shell once Gnome 3 will be released because anyone with CSS knowledge can do it: it's all CSS and images.
Smooth Inset - download and installation instructions @ DeviantArt