1UP writes: "It's Sony's Western-developed PSP game, a Resistance sidestory that takes place between the first and second PS3 games, features a new main character, and has a third-person over-the-shoulder aiming perspective. It also includes a unique control scheme, which you can read more about in our E3 preview.
Quite a lot, actually. In a press breakout session, two members of the Sony Bend studio revealed new features based around connecting the game with Resistance 2 on PS3. If you have each game running on its own system and plug the two into each other, you unlock two options: "infect" your PSP or PSP Plus mode."