Those of you who want to kick up some dirt on the go, here's one for you: U.K. videogame retailer Game have listed a PlayStation Portable version of Sony's racer Motorstorm.
That, unfortunately, is all of the info we have. As with all mysterious retail listings, one can't put too much faith in their accuracy. However, history has shown us that there's a fair chance someone, somewhere knows something we don't. It's also not unlikely that Sony would bring one of their franchises to their portable; see also: Resistance, Ratchet and Clank, God of War, Jak and Daxter, ATV Off Road Fury ... you get the idea.
So far this is nothing more than a rumor, so don't get your hopes up. But if you were to get your hopes up, is this something you'd be interested in?