Since this past weekend in Mesa Git master, which will eventually become Mesa 8.1, R600 Stream-Out is on by default for R7XX GPUs. This is for the Radeon HD 4000 series and it also requires DRM 2.17, which basically comes down to running the very latest Linux kernel (3.5).
For the Evergreen (Radeon HD 5000 series) and R600 (Radeon HD 2000/3000 series) the Stream-Out support has been enabled by default already. Just missing from action now is the Radeon HD 6000 series with the Northern Islands and Cayman GPUs, then of course the Radeon HD 7000 series support too whenever that new driver is finally into shape. The HD 6000 series support can be toggled easily at this time via the R600_STREAMOUT environment variable.
Stream-Out support has been available in the the Radeon DRM driver for months but all of the issues are only now being worked out. Gallium3D has had Stream Out support for since last year. Stream-Out is what enables OpenGL transform feedback support. Stream-Out provides support for storing primitives in a buffer with the data being output directly from a Geometry/Vertex Shader before the rasterization stage