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

Starship Level Complete 
Left, Left, Right, Circle Circle, Right, Right, Left 

Alaska Level Complete 
R1, Circle, R1, Square, Left, Left, Right, Left 

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 

Big Head Mode
Press Circle, Circle, Circle, Square, L1, L1, L1, L2 at the difficulty selection screen.

Turbo Mode
Press L1, R2, R2, Square, Square, Square, Square, L1 at the difficulty selection 
screen

Unlimited stealth:
Press Up(2), Down(2), L1, L2, L1, L2 at the Autobot headquarters menu. A 
message will confirm correct code entry. 

All Mini-Cons:
Press R2, R1, Left, Right, Left, Right, Circle, Square at the Autobot headquarters 
menu. A message will confirm correct code entry. 

One hit kills:
Pause game play, then press Square, Circle, Square, Circle, L1(2), L2, L1. A 
message will confirm correct code entry. Note: This must be done each time you 
enter a new level or leave a level and return. 

Infinite Powerlink: 
UP, DOWN, UP, DOWN, CIRCLE, SQUARE, SQUARE, CIRCLE 

Unlockable: Extra Cinema 
Collect all of the mini-cons to unlock an extra cinema.  

Play as Megatron:
Successfully complete the game under all three difficulties to play as Megatron.

Get Mini Cons at Research Center before fighting Starscream:
Instead of going down between the hangers, walk around behind them. If you do 
not walk in front of Starscream's hanger, he will not come out. Blow up the 
buildings with rockets. Some of the buildings contain Data Cons. Three of them 
contain Mini Cons. One of them can be especially helpful against Starscream, as it 
is the one that constantly restores your energy.

Disable Minicon Overload: 
At the Autobot HQ main menu screen hit the following: R1, R1, L2, R1, R1, L2, 
CIRCLE, CIRCLE 

Invincibility :
From the pause screen hit R1, CIRCLE, CIRCLE, R1, R2, L1, L1, L2  

All extras:
Press Square(2), Circle, Square(2), Circle, L1, L2 at the extras menu. A message 
will confirm correct code entry