This morning on the RadeonHD mailing list, Yang Zhao has proposed patches that add initial PowerPlay support to the xf86-video-radeonhd driver for the R500 series. The R600/700 series is not yet supported or tested.
These patches are based upon the original work of Alex Deucher with an experimental branch he started earlier for improved power management with the xf86-video-ati driver. Powering most of PowerPlay is the AtomBIOS video abstraction layer to supply supported voltages and frequencies that the specific graphics processor is capable of running.
Due to the lacking infrastructure, right now there is no real-time PowerPlay state changing support. To define a power mode to run at, it must be done through the xorg.conf with the PowerPlayMode option.
These patches come with a big warning: "NO GUARANTEES ARE MADE THAT THE ADDED CODE WILL FUNCTION AS EXPECTED. YOUR EXPERIENCE MAY VARY. VOLTAGE AND CLOCK SETTINGS ARE CHANGED BY THE NEW CODE, AND THIS MAY BE DANGEROUS TO YOUR HARDWARE. TEST AT YOUR OWN RISK."
As no PowerPlay documentation has yet to be published by AMD and these patches have experienced limited testing with the R500 series (and the leading focus is R600/700 3D support), don't expect them to appear in the master branch of this Novell-powered open-source ATI driver anytime soon. Should you be interested in these experimental patches they can be found here.