Tho there's probably a way to chip your xbox360 so that it can run windows and therefore cod 4. Although that will mean you can never use xboxlive again, I don't have one but I had mates at school that chipped their original xbox so feasibly possible.
If they made COD4 able to play VS PC, they would have to dumb down the controls, i.e. the mouse would go the way of the Halo cross hair and aiming, to make it "fair" for the thumb controllers. If you want to play vs. 360, get a 360. LIVE for 360, good, LIVE for PC's very bad. Does not allow for mods and server tweaks unless approved by Microsoft.
When playing online there is no reason that a PC can't play Xbox. packets are packets so it does not matter if its from a PC or xbox. One can be in high rez the other can be in low. The server does all the interpetation. So its a server side mod that needs to be done to interconnect them. Then a small patch for access on the gamers side.
So im confused........ can I play with my buddy who has a pc and I an xbox, or not? Lots of technical stuff and no clear awnser. To all of you computer/tech smart people- help this ignorant lout out. A yes or no if you please. Thanx