One of the best GTK themes, elementary, has been updated to version 3.0 today, this being the first stable release that supports GTK3.

GTK3 support, including :backdrop class, but there's no dark theme yet

elementary now uses the Unico GTK3 engine

Granite support (which is used by many applications that will be default in the elementary OS Luna, such as BeatBox, etc.)

Dropped GTK2 classes for GTK3 apps so there's no more support for applications like Nautilus elementary and others (so any application that has a GTK3 version doesn't have support for GTK2 anymore)

The default placement is now close:maximize

The maximize button has been changed from the "+" (typically used to mean new) to two opposing arrows

More

The latest elementary GTK 3.0 has been built with Pantheon in mind and unfortunately it isn't and won't be tweaked for Unity or GNOME Shell so the theme won't look exactly as it should under these desktop environments - for instance, there are no window buttons for maximized windows in Unity, etc. Also, the theme only works with the latest GTK3 (3.4) available in Ubuntu Precise and won't work with older GTK3 versions (like in Ubuntu Oneiric for instance).

Thanks to Daniel Fore for the info!

To install it, extract the downloaded archive and copy the "elementary" folder to thedirectory (if it doesn't exist, create it) for your user or underto make it available for all users (and applications ran as root).