With latest Mountain Lion Beta (3), we have kexts for Radeon 7000 series cards
They are untested at the moment, but netkas does not believe they are fully working yet:
7xxx series will not work just yet, as it needs FrameBuffer driver first, and this is just accelerator
But it seems we will have full 7000 series support at (or before) Mountain Lion release. It is 99% probable that the new iMac will release with 7000 series graphics, and therefore we might even see the support as early as in Lion 10.7.4.