This article at CVG is heavily hinting that may be a possibility. Kazunori Yamauchi had this to say in the latest issue of PSM3 magazine:
Polyphony Digital released Tourist Trophy on the PS2 in 2006 and the game received many good reviews as well as picking up IGN's 'Best Simulation' in their 'Best of 2006' awards.
The environment we have right now allows us to drive motorcycles, but we need to get Gran Turismo 5 done first, Once that's out of the way, if there's a strong demand for bikes, that's something we'd consider
Polyphony Digital released Tourist Trophy on the PS2 in 2006 and the game received many good reviews as well as picking up IGN's 'Best Simulation' in their 'Best of 2006' awards.