Zack Rusin has just shared with the Mesa development community a plethora of new code. This is not code for yet another state tracker or new hardware driver, but rather it brings new geometry shader support to the Gallium3D infrastructure. There's a new syntax now being used for geometry shaders and these patches fully implement the support, including with the TGSI components. So far the feedback on these patches are very positive and should be merged to master in the near future.
See the mailing list thread
for more details.