Terminator 3: War of the Machines Cheats

Start a Game, can be Single Player or Multiplayer, once in, pick any side you want and begin the game, start fragging as many Tech-Com's/Terminators as you can and have the high score in the game at the moment. As soon as you die and go to the Respawn screen, select Tech-Com and switch thru till you see The Terminator (aka Arnold in his T3 Outfit), now you can play as Arnold on the Tech-Com side.

Mini-Games
Centipede: Beat the game to unlock Centipede at the Atari Games menu.

In the vet's office on level 13, walk into the small office on the far right side. Approach the computer and push [Action] to play Missile Command. Then, successfully complete the level to unlock the game at the "Atari Games" menu.