NOTE: THIS IS NOT THE GAME! THIS IS THE GAME SAVE DATA THAT UNLOCKS EVERYTHING IN THE GAME!
This works 100%. I just applied it to my PS3 and now I have all the cars in the game. I only had 14k in winnings/credits though so not sure if that was from before the save or comes on the save.
But all the races are unlocked. A, B, C, S classes are all unlocked and you have ALL THE CARS in your garage!
Enjoy and thanks to "ROGSR34UK" who uploaded the save at the place I found it.
INSTRUCTIONS ON APPLYING THIS:
To install open this zip file on your PC and copy the PS3 folder to your USB memory stick. Insert the USB memory stick in your PS3 and go to the Game menu and select Saved Data Utility. Then move up to the USB Device and select the Gran Turismo 5 Prologue save data, hit your up triangle button on the right of your joystick and then X on Copy. Please note that this will overwrite your existing save game data. I suggest that you install any updates before you copy the save game data as mine is completely current on the updates.