There's no big press release for it or anything right now, but the Southern Islands shader ISA programming guide is available from the AMD developer web-site.
Here's the Southern islands Instruction Set Architecture PDF. The S.I. ISA documentation amounts to 298 pages.
AMD has yet to publish any official public documentation concerning mode-setting or other parts of the Southern Islands GPUs. This initial documentation comes about eight months after the first Radeon HD 7800 series graphics cards began to ship. Meanwhile the open-source "RadeonSI" Gallium3D driver for providing OpenGL acceleration for the Radeon HD 7000 series GPUs on an open-source stack is still being brought up into a usable state for AMD Linux customers.