If you checked out the Ubuntu Quantal blueprints page, you just might notice there's plans for getting Compiz to work with OpenGL ES.
Well, it seems things have progressed quite a bit since the initial plans. The OpenGL ES branch has been merged into Compiz mainline (read more). Basically, we can expect Unity in Ubuntu 12.10 to be using OpenGL ES instead of OpenGL. With this, it should be possible to get Ubuntu running on embedded systems.
Maybe an Ubuntu for Android update is forthcoming? Of course, I haven't heard news on that since the demo @ MWC 2012.