The Transformers Cheats:
Enter the following button codes during the "Difficulty Select" screen to get the desired results:

Alaska Level Complete: 
R1, Circle, R1, Square, Left, Left, Right, Left.
Big Head Mode: 
Circle, Circle, Circle, Square, L1, L1, L1, L2.
Deep Amazon Level Complete: 
Left, Right, Left, Left, Right, R1, R2, Circle.
Earth Level Complete: 
R2, R1, L1, L2, Square, Circle, Square, Circle.
Mid-Atlantic Level Complete: 
Circle, Square, Circle, Square, Right, Left, Left, Right.
Starship Level Complete: 
Left, Left, Right, Circle, Circle, Right, Right, Left.
Turbo Mode: 
L1, R2, R2, Square, Square, Square, Square, L1.

And during game:

Infinite Powerlink: 
Up, Down, Up, Down, Circle, Square, Square, Circle.

Infinite Stealth:
Up, Up, Down, Down, L1, L2, L1, L2.
Note: Must have convert mini-con equipped. 

Kill Any Enemy with One Shot: 
Square, Circle, Square, Circle, L1, L1, L2, L1. 

All Mini-Cons: 
R2, R1, Left, Right, Left, Right, Circle, Square.