I played COD4 seamlessly on my Aluminum MacBook and it was very similar with the 9400m. I was able to play at medium+ settings without dropped frames and great smoke effects.
Granted that was under OS X, windows is different. Someone did report playing MW2 on their MacBook Air with the 9400m and though it was low low res it DID play ok, remember though, the MBP smokes the MBA.
I play Modern Warfare on my 13" MBP all the time with decent framerates and most effects on full. I'd probably get better performance under Windows but I don't like having to reboot just to play thirty minutes of COD.
I'd assume that MW2 would play similarly if under Windows, plus you'd have the upside of playing it now instead of waiting a year for the OS X version.