Im running windows 7 on parallels 5, with setting utilising the majority of my 8 core Mac PRO / 16GB RAM and Modern Warfare 2 is still totally unplayable, you get all excited when the menus load perfectly, but as soon as the game starts you cant even move, a single mouse movement takes about 10 seconds.... so it will be a long time before a virtual machine can handle this sort of application (unless there is a way to utilise one video card for osx, and the other for parallels... gee theres an idea??)
but as for bootcamp... well there is a different story!! Modern warfare 2 works an absolute treat on maxed settings with no lag and ive had it side by side with both xbox and PS3 versions and there is no comparison, the PC version blows them away visually, the detail is greater the speed is better and all without any issues....
I am running MW2 on a boot camp partition and I love it. The game plays and looks wonderful. I have not any issues so far. Many people told me not to even try running these games on a virtual machine.