A quick aside...
As a 360 owner with a long-time love for mouse and keyboard, how does COD4 translate onto the Xbox? I can't imagine trying to line my crosshairs up with the gamepad?
Hard to say really, obviously now it's natural for me just to play a FPS on the 360, I don't find it difficult at all.
But I know a lot of PC owners that can't handle it, whereas others have converted over like me. Takes time, practise makes perfect. You may even prefer it.