iPhone 3GS + iPad here. iPhone is for quick little games. iPad is for same device multiplayer games (Fruit Ninja, Flight Control, Achtung, MultiPong, Jenga, Carcassonne, etc). PSV will fix my 'don't wanna sit on the same couch again' mood. I can freely choose where I'll play my games.
Also, I haven't played any CoD since Modern Warfare 1. I'm probably in for a portable CoD though, seems like a perfect online game if you have say 30 minutes or an hour of spare time. I wouldn't touch any console version of it, but I can see how it would be nice fit on a handheld for me personally.