The Clutter Toolkit is nearing a 1.0 status and it uses OpenGL and OpenGL ES for rendering, but the API exposed to the desktop developer is much simpler. Clutter has a Pango font renderer, Cairo graphics rendering, and G-Streamer support. In particular, Clutter has been gaining ground within the Linux mobile area and will likely be used by the Moblin project. Kristian has created his own Clutter repository and as a branch of it is a Wayland back-end for Clutter.
The discussion about widget tool-kits for Wayland can be found in the Wayland Google Group. Kristian also mentioned another developer had written a minimal desktop shell for Wayland. Porting GTK+ to Wayland is still being planned for the future.