Machinima in Modern Warfare 2

Here's how to do it on the Xbox:
1. Start a Hardcore match. This gets rid of most of the Heads-Up-Display.
2. Make sure your actors have the 'Cold Blooded Pro' perk enabled. This means their username doesn't come up when seen by the player designated as the cameraman.
3. The cameraman should equip a flashbang and hold down the left button. This pulls back the character's arm in the motion of throwing it, but as long as you don't relese the left button the character remains in that position without releasing their projectile. This leaves the screen completely empty and ready for filming.

I discovered step three when wandering aimlessly with Matthieu around Overgrown, and it's essential. It's also useful to note that very different from Halo you'd need a minimum of 2 Xboxes either conected locally or over Xbox LIVE. All the filming takes place in real-time so you can't rewind time to re-shoot the same events again and again.
You also have very little control over the costume of your character.

Despite all that it's not impossible so people will definitely attempt it sometime in the future.