Project Gotham did it. Burnout did it. So if Gran Turismo creator Kazunori Yamauchi is toying with the idea of putting motorcycles into Gran Turismo, you can't say it's without precedent.
In an interview with IGN, the Polyphony Digital boss was asked about the possibility of bikes making their way into the game, to which he responded "If that's what users really want in the game, I think that's a possibility. Or we could consider making a Tourist Trophy 2 as well."
Might be best sticking to TT2. GT5 is late enough as it is.