Up to this point if you are an owner of ATI Radeon HD 5000, AMD Radeon HD 6000, or AMD Fusion hardware and prefer to use the open-source driver to the closed-source Catalyst Linux driver, you have had to rely upon source-code only found in Git. With today's xf86-video-ati 6.14.0 release, the Radeon HD 5000 "Evergreen", Radeon HD 6000 "Northern Islands", and Fusion "Ontario" support is stable. In the xf86-video-ati driver it means the kernel mode-setting support, 2D EXA acceleration, and video playback acceleration via X-Video. The Fusion support provided in this driver update also offers user-space mode-setting for anyone faced by issues with KMS.
For the Radeon HD 5000/6000 and Fusion hardware there's also open-source 3D support available that lives within the Mesa stack. Ideally you should be using the Gallium3D "R600g" driver for the best and fastest open-source ATI driver.
If you're looking to utilize a Radeon HD 5000 series GPU with the xf86-video-ati 6.14.0 driver you will need the Linux 2.6.36 kernel or later. To use the Radeon HD 6000 or Fusion hardware you will need to be using the Linux 2.6.38 kernel, which won't be released as stable for a number of weeks so until then you're left using Git or one of the weekly release candidates.
Taking advantage of ATI KMS page-flipping also requires using the unreleased Linux 2.6.38 kernel. Best about KMS page-flipping is that it dramatically speeds up the performance in OpenGL applications and games.
Also to enjoy in xf86-video-ati 6.14.0 are lots of bug-fixes and a major manual page clean-up. The list of bugs fixed can be found in the release announcement for the xf86-video-ati 6.14.0 driver.