In early April was when AMD finally published the Gallium3D code in the form of a new driver -- the invasive changes of the "Graphics Core Next" architecture necessitated a new user-space OpenGL/Gallium3D driver rather than just extending the existing "R600g" driver. In mid-April, the RadeonSI driver code was merged into mainline Mesa. However, it wasn't exactly usable at that point nor what AMD customers would expect, "Basic egl demos are starting to run."
Now going on six months since the first AMD Radeon HD 7900 series graphics cards launched, the open-source support still leaves a lot to be desired. The "RadeonSI" Gallium3D driver for the Radeon HD 7000 series is still far from complete. At least though, progress is (slowly) being made. The commit activity for this driver can be found from Mesa CGit. The most recent RadeonSI driver commits were just hours ago.
Hopefully there will be some sort of decent support for the HD 7000 series with Mesa 8.1, which is due out this August. But for now the only real option is to use Southern Islands with the Catalyst driver.