Somehow, running Modern Warfare 2 coincidentally fools the system into thinking it's authorized to temporarily unlock full versions of Garou: Mark of the Wolves, The King of Fighters '98 Ultimate Match, Metal Slug 3, Metal Slug XX, and Samurai Shodown II. Joystiq tested the exploit with Garou and KOF '98, and can verify that it works. For now, at least.
We're certain this issue will be patched by one of the three affected parties -- SNK Playmore, Activision, or Microsoft -- in the near future, now that it's known. In the meantime, just because you can get these games for free doesn't mean you aren't a major jerk for doing so.
[Note: This doesn't permanently unlock the games; only if you launch them directly from inside MW2. If you launch from the Dashboard, they'll behave just like plain ol' trial games.]