We're not sure whether Gran Turismo 5 Prologue is meant to be a demo, a hype-builder or some sort of mid-development cash-in for Gran Turismo 5 (probably all three), but we do now know that it will be made available in America on April 17th. The game can be purchased on a retail Blu-Ray disc or as a PSN download, just like Warhawk. Whichever way you buy it you'll be paying the same - $39.99, though the Blu-Ray version will come with a HD behind the scenes video titled "Beyond The Apex".
The 60+ cars included can be raced online with up to 15 other people. Polyphony Digital will be holding official online races and Gran Turismo TV will allow you to watch motoring shows from all over the world via the PlayStation Network. Not bad for a demo, even if it does cost $40.