Those interested in this new document release, it's available at AMD.com (PDF). It's just under 500 pages and is separated into program organization and state, control flow programs, ALU clauses, texture cache clauses, memory read clauses, data share operations, instruction set, and microcode formats. This documentation though does not cover mode-setting or other non-ISA portions of the graphics processor.
Aside from the Radeon HD 6900 ISA documentation, AMD's Alex Deucher also has reported to Phoronix that the open-source support for these GPUs is coming along as well. "In the open driver, we have working mode-setting support for Cayman and much of the acceleration work is in place, but it's not working yet."
This is good news so it looks like we may see Radeon HD 6900 series open-source support land for the Linux 2.6.39 kernel with the DRM/KMS Radeon bits and then in Mesa 7.11 for the user-space Gallium3D driver support with OpenGL acceleration. The 3D support is expected to be tacked onto the "R600g" Mesa driver.
For those owners of pre-HD 6900 series hardware, as in the Radeon HD 6000 "Northern Islands" ASICs and previous generations, there is already open-source support. Via the proprietary AMD Catalyst driver there is already support for all Radeon HD 6000 series GPUs, including the Cayman-based Radeon HD 6950 and Radeon HD 6970.