Kotaku's liveblog of this Sony event has some details/news on GT5. It will be out March 2010 in Japan with no word on other territories. There will be voice and text chat for online play and Youtube recording/uploading. They also mention something about exporting videos to PSP, though that's probably just using the export to menu option and then transfering that to the PSP directly.
Edit: G4's liveblog says there are 950 cars, 20 locations, and 70 tracks and details on features or menu options, whatever these are supposed to be:
Physics: new physics, damage, movable objects on the track, and the newest hybrid and electric cars.
Arcade Mode: Single Player, split-screen two player
GT Mode: World Map, My Garage, Car Dealer, Tuning Shop, Championship Race, and License Test.
Online: Open Lobby, Text/Voice Chat, Private Room, Online Photo Album, Online Replay Album, and you can export replays to YouTube.
Photo Mode: Photo Drive, Photo Stage
GT TV: Improved UI, export of videos to PSP, and progressive downloading.