Polyphony Digital's Kazunori Yamauchi was interviewed by Autoweek on Gran Turismo 5, and during that interview he was asked about the next Gran Turismo. And instead of blowing the interviewer off, Yamauchi spoke freely about Gran Turismo 6!
Says Yamauchi when asked about the next GT game:
I have no idea what the hell he was talking about with the operating systems and stuff, but I did like the part about not having to wait as long. I know it's too soon to be thinking about this, but this definitely points to Polyphony Digital at least thinking about the next game. Hell, working on it now before its predecessor is released will surely cut back on some of the ridiculous delays that we should expect.