Now here's an odd one for you. There's a glitch in the full and demo version of Modern Warfare 2, though to be fair, the glitch is really with SNK's Xbox Live Arcade titles - where you get free access to the full version of trial titles from SNK that are installed on your hard drive. This includes all of SNK's games, including the co-op enabled titles like Metal Slug 3 and Metal Slug XX.
To do this all you need to do is fire up Modern Warfare 2, hit the guide button and go to the quick launch tab. Now select one of the SNK titles and launch it. Voila, full version. The effect is only temporary though and quitting the game will reset it back to the demo state.
Expect a patch from SNK shortly. SNK - expect a check from Bobby Kotick shortly.