Zack Rusin has done some work previously on supporting OpenGL geometry shaders within Mesa/Gallium3D, and now Bryan Cain has been building upon this code to support the ARB_geometry_shader4 and EXT_geometry_shader4 extensions. OpenGL geometry shaders are one of the big features to the OpenGL 3.2 core specification. Geometry shaders control primitive processing and can generate new primitives; this post vertex shader process was added to the OpenGL 3.2 core but has been around earlier through extensions.
In Cain's GitHub geometry-shaders branch since yesterday he now has all the GLSL demos that utilize the geometry shader extensions working fully. However, right now it's only working with the Mesa Softpipe driver and not any of the actual Mesa/Gallium3D hardware drivers. There's also some other work he says that should be completed prior to this being merged to master, so it might not make the cut for next month's Mesa 8.1 release.
The hopeful Mesa geometry shader mailing list message can be viewed from the archives.
More information on OpenGL geometry shaders is available from the OpenGL.org registry.