AMD's Alex Deucher shared these latest patches last night that go against the Radeon DRM and the xf86-video-ati DDX for providing this page-flipping support. The announcement was made here and require using the drm-next tree plus David Airlie's 2D tiling fix patches. Alex is reporting that the page-flipping is working out well for Radeon R600/R700/Evergreen GPUs (the Radeon HD 2000/3000/4000/5000 series), but earlier R100 through R500 ASICs still need additional work.
Unfortunately, it doesn't appear as if this work would be ready before the merge window for the Linux 2.6.37 kernel closes. The page-flipping still isn't working well in tiling scenarios and as aforementioned the support for earlier GPUs needs some additional attention. As such, we may not see mainline ATI Radeon page-flipping support with the open-source driver until the Linux 2.6.38 kernel. At least we have XvMC on R600g support to play with for the time being.