Ive just come back from seeing Terminator Salvation, and have thought up some ideas.
1. I was framed for a crime that i didnt commit, sent to death row,and was offered the choice that Marcus was.Perhaps i was slated to be the back up in case he failed.So i wasnt intended to be let out of the lab, but i break out after most of the base is destroyed and seek out the resistance.
2. Im a new commander, are captured by Skynet and have an android body created in my image.My memories are transferred to my new android body, my human body is destroyed. I would break out after an attack and reclaim my life, not realiising what i am, or perhaps i do know.
3. Im a pilot or regular soldier in the resistance, that can go a few different possible ways.