Programming these new ATI graphics processors are quite similar to the previous-generation R600/700 series (Radeon HD 2000/3000/4000 series) so the plan all along has been to tack the support onto the R600 Mesa driver. However, due to different register offsets with the "R800" (Evergreen) hardware, this is looking to not be the case. Instead AMD is now looking at basically copying the R600 driver and calling it "R800" (or something similar) and then from there gutting out the different registers while keeping most of the code in place.
This will mean that much of the code and fixes will now need to be replicated across two classic Mesa drivers, but due to this unplanned change, it may be an opportune time to jump-start work on the Gallium3D upbringing for the Radeon HD 5000 series graphics cards. The DDX driver will also have problems with different register offsets between R600/700 and R800, so AMD may also take this opportunity to push for the 2D EXA and X-Video acceleration via Gallium3D.
John mentioned this change on the mailing list and also in our forums, where he is often addressing the ATI-related questions of those within the Linux community.