In an interview with Eurogamer’s Rob Fahey, Kazunori Yamauchi (the creator of Grand Turismo) revealed that they are still working on the PSP port of the PS2 game. On the other hand, he reveals that it won’t be done by the end of this year.

From the article:

Eurogamer: In the process of developing GT5, have any of your thoughts on the PSP version changed?

Kazunori Yamauchi: That’s definitely true. Once we experienced PS3 online and went through all of that, we came to the conclusion that PSP should not be a standalone product – it should be linked in to the world of Gran Turismo, linked with the PlayStation 3.

Source: Eurogamer (Page 2)