This is a good idea in theory. If I had any hope that the studio behind the 5th Terminator film could give us a decent story with a reasonable explanation for why an android who shouldn't age looks decades older, then I'd be cautiously optimistic. But there's a problem. We have yet to see a reboot of a grand old sci-fi property from the 70s or 80s that didn't get screwed up royally by screenwriters and directors just looking for a quick buck. In the last 15 years, we've had to endure the Alien vs. Predator flicks, the Transformers franchise, the Star Wars prequels, and of course Terminator: Salvation.
With the right director and the right script, this could work. I repeat, could. I'd love to see what J.J. Abrams would do with the franchise, or Joss Whedon perhaps. The only name they're throwing around right now is Justin Lin, who most recently directed Fast Five. Yes, the future of SkyNet could be determined by the man who directed the fourth sequel in the Fast and Furious line of movies. Oi. (Although to be fair, Fast Five has far better reviews than anyone predicted. Still...)
I believe that if handled well, Ahh-nold would deliver. I've always liked the man's performances, even when they're not very good. He was perfect for the stern, deadly serious T-800, and if they thought of a good explanation for why the robot is now a senior citizen, he'd do the script justice. But they won't, and he shouldn't. This is a bad idea, simply because when it comes to successful reboots of beloved sci-fi franchises, Hollywood misses the mark 99% of the time. Don't do it Arnold, don't do it.