This is a bit of a weird one, especially since Modern Warfare has absolutely nothing to do with SNKP, but i got a message from my friend this morning before work telling me that the demo for Modern Warfare 2 causes a 'free game' glitch for certain SNK games.
What appears to be the case is that booting up the demo for MW2 and then quick launching an SNKP trial title (only 5 are affected; KOF98UM, SamSho2, Metal Slug 3, Metal Slug XX, and Garou:MOTW) grants access to the full SNKP game.
I'm not sure how this has happened and what caused it, but i did manage to test it before work and can say that it did in fact work for Metal Slug XX. Even though you can play the full game, it does not give you the game permanently.
Strangely it appears that it is only these five SNKP titles that are affected, i've not yet heard of any others having this problem.
I doubt this glitch will last forever though.