This is follow up to our yesterday's post about the new Unico GTK3 theming engine in Oneiric. I will skip providing instructions on installing the new Ambiance theme (based on Unico engine) as you will have to tweak lots of system settings using gsettings and dconf-editor. Also I think, sooner or later you will be able to change most of these settings from a GUI.
But if you really want to try this stuff, you can follow this thread on Ubuntu Forums for instructions. Keep in mind that Ubuntu 11.10 has not even reached Alpha 1, so you should not try anything on your production system.
Adwaita Theme Screenshots
Ambiance Theme Screenshots
Lucazade is working on the Ambiance theme. The theme is in initial stages of development and lot of work has to be done.