For the past month this AtomBIOS-ized version of the xf86-video-radeonhd driver have lived in a separate development branch, but today more than one hundred patches have been merged to master. Now by default the driver will be using AtomBIOS.
A number of the R500 and R600 graphics cards though have hard-coded mode-setting support that was done over the past year, but not all of that work has gone to waste. Where there is a hard-coded path, the RadeonHD driver will still use it unless the user sets the UseAtomBIOS option within their xorg.conf. For all other chipsets, AtomBIOS is used by default.
Next up will be to merge the HDMI audio and Command Submission driver branches to master.