In my lastest commit to my branch (r28848), I got geometry shaders working in the viewport and the BGE. Here is a screenshot of a tessellation shader I got from Martinsh (thanks Martinsh!):

If you look at the right 3d view you’ll see that the plane only has a single face. However, in the left 3d view, you can see the effects of the geometry shader, which tessellates the mesh (adds more geometry).

For the time being GL_GEOMETRY_INPUT_TYPE_EXT is set to GL_TRIANGLES and GL_GEOMETRY_OUTPUT_TYPE_EXT is set to GL_TRIANGLE_STRIP. I’ll add the ability to change these later. Also, soon I’ll be working on passing uniform values around.