Published on the Radeon driver mailing list this morning is AMD's support for multi-screen reverse Optimus. Like the Nouveau code talked about earlier, this support is for allowing multiple screens to be powered in a reverse Optimus configuration: where the dedicated GPU is driving multiple displays but the rendering is being powered by the integrated GPU (generally, Intel graphics). There's a new X.Org Server interface to allow for multiple outputs on the dedicated GPU with the integrated GPU being responsible for the rendering.
With the port from the Nouveau code and taking advantage of a new X.Org Server interface, the code to the xf86-video-ati DDX is less than 100 lines. The two patches for the Radeon support can be found on the xorg-driver-ati mailing list.