The idea of OpenGL widgets has always appealed to me. The main reason is consolidating on a single method for drawing and the fact that I could finally get true alpha-channel images. Back when I was trying to do Enlightenment themes, for the contest that never happened, I felt that it would look better if I could get the snow to actually look good instead of hard ice on the window borders. Of course, at the time (1999), that wasn't possible.
So, I've been occasionally paying attention to those attempts to make an OpenGL widget set. There was Berlin, which I thought had potential, but it never seemed to go anywhere. And with the Xgl that came out, there is a chance they could get something very close (not unlike the composite manager) that would work. I tried to do something with my UTTIL library and eventually the gtk-trans (transparent/translucent Gtk theme), but it sputtered with my lack of understanding of garbage cleanup. On the other hand, having translucent skinned buttons and windows in 1999 was cool.
Of course, I haven't done much with widgets since the little library for SDL.NET that never went anywhere. So, I don't know the complexity, but I do know that if it requires yet another library, there will be a war. I also wish that the Qt and Gtk worlds would work together. Even if it was just a common widget language that called either/or without all those annoying "my widgets are better" type of argument.
So, the idea of an OpenGL widget set or base for Gtk (or anything) will be something I will pay attention to. It holds that promise of things I'm "interested" in in the future.