That doesn't mean you should treat this as fact, just know that here at PushSquare we trust GTPlanet as a source.
While hesitant about giving concrete details (for credibility's sake), GTPlanet are reporting that they've been tipped off regarding the tentative release date for Gran Turismo 5 and it hits this year. According to the site the date will be announced as a part of Sony's three hour GamesCom conference in Cologne this August 18th.
The smartest move for Sony would be a Christmas bundle of the rumoured PS3 Slim and Gran Turismo 5. Note: it could happen.
Don't get that hype meter flowing until Sony make an official announcement though.