A new feature named PSP Plus (can be found in games like Resistance: Retribution) will allow owners of both the PSP and PlayStation 3 to be able to control the PSP with the Dualshock controller.

If you have a PSP 2000 or 3000 just connect it to your PS3 via USB and you’ll be able to switch on PSP Plus control through Resistance 2.
That’s right – you have to have a copy of Resistance 2 in order for this to work.

PSP Plus lets you play PSP games on your television with the Dualshock controller. It disables aim assist in Resistance: Retribution and allows the player to enjoy the controller’s extra inputs.

Source