To be found in drm-radeon-testing right now is new I2C code that supports the hardware I2C engines found on Radeon graphics cards and exposes it to user-space, a PLL algorithm rework, DRM power management support, basic Evergreen "R800" KMS support, and various other fixes and new additions.
Like the recent R800 Evergreen in xf86-video-ati DDX support, the kernel mode-setting support lacks IRQ and any acceleration (2D, 3D, X-Video) support at this time for these Radeon HD 5000 series products. The new I2C support can be in turn exploited for fan/thermal monitoring on select ATI graphics cards, among other uses.
The list of what can currently be found in drm-radeon-testing at the moment is shared on David Airlie's blog. Clone the drm-radeon-testing branch of git://git.kernel.org/pub/scm/linux/kernel/git/airlied/drm-2.6.git to fetch the code.