The extension that Ian Romanick and others at Intel have been spearheading is GLX_MESA_query_renderer, that article covers the details and use-case for the new GL extension to assist game developers.
In a mailing list post Thursday, Romanick says he is still working to land the extension but hasn't received any developer review. However, he's pushing the open-source AMD driver developers to review the code and to ideally implement support for GLX_MESA_query_renderer within their R600/RadeonSI drivers too. Right now only the patched Intel DRI driver is exposing this proposed extension.
As of yesterday, Ian provided a Piglit test for verifying the GLX_MESA_query_renderer extension and he's also planning to update the glxinfo utility for being able to utilize the extension with its OpenGL information reporting. Hopefully we will be able to see this Mesa OpenGL extension land in time for the Mesa 10.0 release in November.