For all the cheats below, first goto the Options Menu and select Credits. Then select Developers. After the FMV has finished you should follow the directions below:

Unlock Treasure Island:
While holding L2 enter the following:
Square, Circle, Square, Up, Up, Down, Up, Left, Right, X

Unlock Proving Grounds Track:
While Holding R1 enter the following:
Left, Circle, Up, Down, Right, Right, Right

Unlock "Shorty":
While holding R2 enter the following:
Square, Circle, X, Up, Down, Left, Right

Unlock K.C. Monoxide:
While holding L2 enter the following:
Square, Circle, X, Down, Up, Right, Left

Unlcock "Jocko" Michaels:
While holding L1 enter the following:
Square, Triangle, Square, Circle, Square, X