It looks like that PS3 to PSP connectivity will add quite a bit of features to those of you who own Resistance 2 for the PS3 and buy Resistance Retribution for your PSP.
This video walks you through some of the features which are broken down into two different modes:
Infected mode will allow you to connect the PSP shooter to the PS3 shooter and infect the main character with the Chimeran virus. Once infected the lead will have a new look, new abilities, new weapons and the game itself will get new dialog because people will react to your infection.
Plus mode will let you use a Playstation 3 controller to play the entire game on your PSP. That means dual analogue sticks, and because of that the game will be made a bit harder and, of course, aim assist will be turned off.
I love how ownership of both games gives you two new ways to play the game, though it would have been nice to support the PS3 controller option for everyone, not just people who own both games.