This question has been asked a million time before. The truth is, nobody knows, except for Arnold of course. Since he isn't talking, it does us no good to guess.
Oh, and it's great to hear that wasn't Arnold in T3. I'm so shocked! They must have built a robot double! He was doing a special ET live on the set interview about 6 months back, and he was wearing a tight shirt, and had a small waist, huge arms and pecs again. Those on the set pics of him doing a double bi shot in Flex were fake too.
Face it, the man has better genetics than you could ever dream or hope of having. With muscle memory, some hrt, and 3 months in the gym, it's not hard to believe he got back into his movie shape again. It's not like he looked like he did when he won the Mr. Olympia.
As for him being out of shape when it's not time for a movie, cut him a break. He is 56, and owes nothing to you, me or the guy down the street. He's won it all, and done it all. Now go back to the gym with your 12inch pipes and do some cable curls, while you drink some nitrotech. (not directed at the starter of this post, but the two haters.)