Due to significant architectural differences between GCN (Graphics Core Next, a.k.a. the Radeon HD 7000 series) and the Radeon HD 2000/3000/4000/5000/6000 series, a new user-space 3D driver is being developed. The "R600g" Gallium3D driver currently supports from the earliest Radeon HD 2000 graphics cards up through the latest Radeon HD 6000 series (Cayman and Northern Islands) GPUs, but it's not being extended for the Radeon HD 7000 series.
The Radeon HD 7000 series support will come from a new Gallium3D driver, which is being started from a stripped-down version of R600g.
In response to the earlier AMD Radeon HD 7000 series Linux story this week, John Bridgman commented some more. His critical new comment is "The current plan is to start a new driver for GCN, based on a stripped-down copy of the r600g code."
So there's going to be yet another graphics driver to work on and maintain in the open-source world. AMD's other Gallium3D drivers are R300g for supporting the higher-end Radeon 9000 series GPUs through the Radeon X1000 (R500) series and the aforementiond R600g driver for the Radeon HD 2000 series through the Radeon HD 6000 series. There's also the vintage R100/R200 drivers that are built on Mesa's classic DRI architecture.
My hope is to see a working open-source Radeon HD 7000 series stack with 3D acceleration by this summer and it being ready for initial integration into Linux distributions by late Q3 of 2012. For the proprietary Catalyst Linux driver, there will be support at launch for the next-generation GPUs. Plus there's my earlier comments from this week.