Sony's Bend Studio revealed at the TGS 2008 some interesting tricks the upcoming Resistance: Retribution for the Sony PSP and the Resistance 2 Game for the PS3 can do together.
You can "infect" your PSP by connecting the PSP to the PS3 running each Resistance and then select "infect your PSP". In infected you get an alternate story. While in prison for desertion, Grayson is recruited by Specter Lieutenant David LaSalle (from the R2 storyline) and is infected with a mutated version of the Chimeran virus; a different version of the virus that infects Nathan Hale. While playing in infected mode, Grayson wears a Specter uniform and has glowing Chimeran eyes.

Additionally the Resistance games offer PSP Plus! which allows you to play Resistance: Retribution using a Dualshock 3 wireless controller on your PSP. The aim assist is removed though and the challenge level is increased while using the Dualshock controller.

These two features are an excellent motivator for PS3 fans to go and buy a PSP as well, if they did not already. Sony does a great job on finding synergies between the two. There are already several features on the media side that have the two consoles work together. Now there are also attractive shared game features.
Via the Sony Playstation blog. See also Resistance: Retribution for the PSP on Amazon.com.