Please Implement Steam VAC as soon as practicable...

[jimbop]

Don't know is the short answer but I understand it checks file integrity (could be wrong or incomplete): http://en.m.wikipedia.org/wiki/Valve_Anti-Cheat

The risk with enabling VAC is that non-cheaters would also get banned. Given the history of CoD development this would not be surprising and I think the devs are wise enough to realise this. I seem to remember Luthier saying something similar a few months ago, that VAC would not be enabled until the game was stable.