I think you've gotta remember that Halo 3 was only available on Xbox 360, whereas COD-MW2 is a cross platform game(XB360,PS3 + PC). Considering that Halo 3 reached 170million on one console only is massive.
I think the next time we'll see this record go is with Modern Warfare 3, the Halo franchise proved you can go bigger and better with Halo2/Halo3 and I think Activision will already be rubbing their hands with glee.