A new bit of info: -
A very credible poster over at the GTPlanet forums posted today about GT5. Of course, it is rumor mill for now, but the poster, Amar, has been right before. From what we can deduce, GT5 should have a worldwide release in the next 12 months, possibly this December. The game will feature over 600 cars, 96 tracks including GT4 remakes, and a track editor where you can splice different parts of existing tracks together. The game will support clan plan, voice chat, and a party mode. There will be two release versions, On where you will pay a large amount of money that allows free access to all future DLC, and one where you have the option to pay a smaller amount and pay for the future DLC individually.
* Worldwide release - December (maybe)
Over 600 cars
96 tracks including GT4 remakes
Track Editor - splice your own track together - Holy
Clans
Voice Chat
Party Mode
Other rumours:
Changeable weather
Damage to Race cars only