Looks like the new GLX_EXT_buffer_age extension has landed in the 313.09 beta drivers. From what I understand, if the composited window managers implement this mechanism, it should really improve tearing issues. I think GNOME and Kwin are already working on it.
It would be nice to resolve this problem once and for all.
Though now that undirect fullscreen windows is working well in compiz, I haven't noticed tearing in fullscreen games anymore.