Gnome 3 stack is steadily landing into Oneiric. Work is also being done to port default themes Ambiance and Radiance to GTK3.
Light themes in Oneiric will most likely use Unico theme engine and not Murrine as some style guidelines for GTK3 themes have changed. Unico was actively developed in past but the work stopped as the new overlay scrollbars in Natty took precedence. Unico engine is already present in Official Ubuntu 11.04 repositories but that should be only meant for testing purposes as it is far from being finished. However, the development has resumed now.
Unico Description in Launchpad
Unico is a Gtk+ engine that aims to be the more complete yet powerful theming engine for Gtk+ 3.0 and newer. It’s the first Gtk+ engine written with Gtk+ style context APIs in mind, using CSS as first class citizen.
No more information about Unico is available as of now. The package has been uploaded to Oneiric repositories only two days back.
A new Ambiance theme (work-in-progress) which is based on Unico is available. I will post installation instructions with some screenshots tomorrow after testing it on one of my systems as I am travelling yet again and will reach my office tomorrow morning. So if you are using highly unstable and far from complete pre-alpha of Ubuntu 11.10 on a system which is NOT your production machine and are willing to get burnt, stay tuned.