The GL_ARB_base_instance extension allows for the offset within buffer objects used for instanced rendering to be specified. Thus this extension is dependent upon the GL_ARB_base_instanced extension. For those wanting the full technical details on this OpenGL extension, see the OpenGL.org registry.
The patches from KDE's Fredrik Höglund for providing the GL_ARB_base_instance support in Mesa/Gallium3D can be found on the mesa-dev mailing list.
It's too bad though that overall we're still a ways out from seeing OpenGL 4.0/4.1/4.2 compliance in Mesa. Full compliance with OpenGL 3.1 isn't even expected until next year.