Extending R300-style 3D support to R5xx seems to be pretty straightforward -- the 3D engines are very different internally but have a very similar programming model. The first use of the R5xx 3D docs was for Textured Video support, which was a surprise but which also seemed to make for some real happy users.
Since the R300 3D was largely reverse engineered the first step seems to be cleaning the code up a bit using the information we released recently :
I think it will make sense to extend R300-style 3D to R6xx as well but we'll know better in a couple of weeks.
There seems to be pretty strong agreement that extending the current R300 mesa code base to GL2.x or higher is not a good idea -- for any work like that, building on Gallium seems like the way to go.