The AMD Radeon HD 6950/6970 "Cayman" graphics cards have been supported by the proprietary Catalyst driver from day-one, complete with acceleration and the expected feature-set, but the open-source support was months behind. Only this afternoon is there a stream of patches hitting the open-source ATI driver tree for finally flipping on the acceleration.
In the xf86-video-ati DDX Git tree are Cayman-specific enablement patches: enabling all acceleration, bringing up EXA/X-Video acceleration, and adding the 3D register headers, among other commits.
The Cayman architecture is quite different from the other Northern Islands graphics processors (i.e. Turks, Juniper, Barts, and Caicos), which resulted in the Cayman open-source upbringing taking much longer for AMD. Even with other Radeon HD 6000 series graphics cards, the latest code is still fairly problematic for me.
Fortunately, AMD's bringing on more open-source developers and with the Radeon HD 8000 series they should finally reach some open-source parity whereby the new ASICs will be supported in the open-source stack on launch-day, or at least much closer than it is now for AMD. On the Intel side they largely manage to do this, sans bugs and getting it back-ported to already in-use Linux distributions, while the Nouveau support obviously takes a while as well due to the burden being placed on the community in reverse-engineering the NVIDIA blob and then writing up an open-source driver.