Speaking to CVG, Polyphony Digital boss Kazunori Yamauchi's confirmed that a PSP version of Gran Turismo is still on the way but it's going to be a GT5 add-on.
"I don't think Gran Turismo Mobile will make this year," he said, "but we are hoping to make the PSP version a satellite game, like an addition to GT5."
GT5's about a year off, so it's going to be a while before you're fiddling with Yamauchi's motor in the palm of your hand.
There's a full interview through that link. Apparently Eurogamer's got one going up today as well, so watch out for that.