Sony is adding rumble support to the PSP, courtesy of the DualShock 3. Using the new "PSP Plus" feature in games like Resistance: Retribution, owners of both the PSP and PlayStation 3 will be able to forego the portable platform's analogue nub for a better control experience.
By plugging in your AV-output capable PSP (2000 or 3000), you'll be able to switch on PSP Plus through Resistance 2. Doing so adds DualShock 3 support, disables aim assist in Resistance: Retribution and lets the player alter their control layout to take advantage of the PS3 controller's extra inputs.
The option requires that gamers have a copy of Resistance 2 in their PS3s, but will not require a firmware update for either the PSP or PS3.