To enable cheat mode, activate your PDA and press L2, R2(x2), L2, R2, L2(x2), R2(x3), L2. Press back. Activate the following cheats by entering the corresponding sequences of buttons. You must have the Oct. 23, 2008 online update for these to work:

CHEAT FUNCTION - CHEAT CODE
All Airstrikes* - RIGHT, LEFT DOWN, UP, RIGHT, LEFT, DOWN, UP
All Costumes* - UP, RIGHT, DOWN, LEFT, UP
All SUPplies* - LEFT, RIGHT, RIGHT, LEFT, UP, UP, LEFT, UP
All Vehicles* - UP, DOWN, LEFT, RIGHT, RIGHT, LEFT
Fill Fuel* - UP, UP, UP, DOWN, DOWN, DOWN
Infinite Ammo* - UP, DOWN, LEFT, RIGHT, LEFT, LEFT
Invincibility** - UP, DOWN, LEFT, DOWN, RIGHT, RIGHT
Nuclear Bunker Busters(25)* - UP, UP, DOWN, DOWN, LEFT, RIGHT, LEFT, RIGHT
Unlock Grapping Hook** - UP, LEFT, DOWN, RIGHT, UP

*Activates only for the player that executes it.

**Activates for both players during co-op. Activates for a client who joins a hosts game that has it unlocked.