Not sure why people are comparing favorably to Valve. I mean hacking in CS:S and TF2 is still pretty rampant. I'm not sure about LFD1/2 but there seems to be little need to 'hack' on those games as you can grief pretty easily without hacking :P
It's unfortunate there isn't any 3rd party plugin for hacking detection. I think this kind of things might be something that comes to the fore front soon. We already have 3rd party graphic engines, physics engines, particle effects, facial animation, etc etc. I'm surprised no one has stepped up to fill in this pretty obvious void. I think it would be pretty popular as a 3rd party plug in for developers. Also having a company dedicated to it, would mean they can bring experience that most devs probably wouldn't have, freeing up resources for other activities.