Alex added basic power management code to master, so I think the branch can be considered obsolete.
If you pull the latest -ati code and check the man page there are two new options - one forces a low power mode by dropping engine clocks and reducing the PCIE lane width, the other drops power draw further when the display is blanked by DPMS. Also, the DynamicClocks option was renamed to ClockGating, or something like that.
Similar options have been added to radeonhd, although I don't think the PCIE lane reductions were ported across.
BTW "PowerPlay" is the name of our proprietary power management software in the Catalyst driver, so we should probably use some other term for the open source drivers to avoid confusion.