So this is why Gran Turismo hasn't come out for PSP: Sony has a much better idea for "realistic" PSP racing. Siliconera found a patent filed by SCEE for a remote-controlled car that uses the PSP as an interface. The car is equipped with a camera that feeds video back to the PSP. One possible application: "a user may control the car... so as to spy on their friends... then upload the stored video images to a website."
In addition, the patent application states that the car could be used for an "augmented reality" racing game in which the game recognizes real objects as elements in the racing game. Basically, you build your own track from stuff around the house.
The only thing we can't figure out about this patent is why it's not a real product yet. Why is nobody selling us this right now? If Sony really wants the PSP to succeed, here's how to do it. A pink PSP and a Hannah Montana game aren't going to be any significant competition for the DS. But a remote-controlled spy car is, because that is awesome.