At the 'Press Start' screen, hold L1 + L2 + R1 + R2 and enter the following:

Unlimited Health:
Up, Right, Down, Left, Up, Left, Down, Right

Unlimited Combo Meter:
Left, Right, Up, Down, Right, Left, Down, Up

All Upgrades
Up, Up, Left, Left, Right, Right, Down, Down

End-Game Rewards:
Down, Up, Down, Up, Left, Right, Left, Right

Dark Knight Difficulty
Down, Left, Right, Up, Up, Right, Left, Down

Thanks to Revolution readers JD and Oranjuice!