When I decided to hack on radeonhd to improve something I found:
which have me some idea how modesetting is made.
Unfortunately I can't find any such a nice howto on 2D, Xv or 3D. I've heard many times "read the driver/code" and I agree it's kind of way of learning, but... not without any knowledge.
When I read /HowVideoCardsWork/ I knew what is VRAM, CRTC, PLL and driver code made sense to me. It's hard to read it without basic knowledge. You don't even know where to start.
It would be nice if you could guys think about some documentation on how 2D/Xv/3D works. This won't give you any new developer in a months, but maybe some ppl will able to produce single patches for little issues.