I fully expect a mid-late Spring 2009 launch in Japan. However, it will not be as feature-rich as they and fans would have hoped. Instead, they will get the technical infrastructure around the core gameplay down-pat, and then rely on downloadable content over the months/years after launch to expand the content. That will be it I think on PS3 - no true sequel to GT5, just lots of extra content in the form of vehicles, tracks, game modes etc., and tweaks to physics and damage modelling etc. over a period of time.
I woldn't even be surprised to hear that owners of GT5P will not have to purchase the "final" copy of the game, and instead have the option to just download another big update.