Lotsa interesting mail on the Cairo list lately. Someone CC'd the Mesa list and there're even some comments from Brian Paul (how many names have I memorized only from their association with software?). Apparently the guy who's been doing the OpenGL backend (some performance numbers) for Cairo is the author of the Waimea window manager, too. He has some screenshots using Cairo for window decorations (if that's with GL, that's hardware accelerated!) and apparently using the xorg server (fancy shadows on the last shot).
With some trickery, you can even implement atoms with property lists, type safe but dynamically typed of course. See the code below. (Warning: the implementation of properties is a real mind-twister, using a folklore trick to implement dynamics using references and functions that I learnt from Daniel de Rauglaudre, although it's been reinvented independently quite a number of times.)http://caml.inria.fr/archives/200105/msg00175.html