Salvation is just a standard hollywood "revive old series" flick... It's fine on it's own for what it is, but honestly, it did not capture the atmosphere etc that made T1 great at all, and there were a number of other things wrong with it...
If they can get the overall feeling right again, then I'm all for Arnie coming back etc. Like many oldschool guys (Larry Scott looks or at least a few years ago looked better than when he was competing, Nubret is dead now but he looked phantastic until he had his stroke or whatever it was... I wish he hadn't tried to get into contest shape at his age...) Arnold has repeadedly demonstrated that he can get into great shape even in his old age.
I'm only a bit worried about his heart condition... Hopefully he'll be alright and won't try to get too lean for the sake of the movie.
And hell... They can just dress him up in a leather jacket as usual (he had nowhere near the size he used to back in the day in the old films anyway) and use some CGI if necessary like they did in salvation... Worked well enough.
But Arnold or no Arnold, I really do hope they change the overall atmosphere and feeling of the movie. And please no Christian Bale. The guy so isn't Connor it's not even funny. He's great in other roles, but please, never again in Terminator.