Arnold Schwarzenegger has given a hint about his much-rumoured cameo in Terminator: Salvation during an interview for an on-line blog, according to The Los Angeles Times : "Schwarzenegger told blogger Bill Bradley, who asked some questions of his own and others submitted via Twitter, that he had met with the director and agreed to be in the movie as long as he didn’t have to perform. ‘I made it very clear that I don’t have the time to do the movie,’ he said. ‘I said that I would be willing to be in the movie if they get the technology together, and so they are working on that right now.’”
However, director McG has a slightly different story to tell over at Comic Book Resources : "Truthfully, it’s not clear. It’s not clear yet so we’ll see where we land. He’s a very powerful guy and he’s got to do what’s right for him. You know what would be helpful is if all of us collectively put a little pressure on him in a couple of emails by saying, 'Hey, we think it would be a good idea.' I mean, we’re not getting any blowback or resistance, I just have to respect that the guy is the Governor of the state of California. He’s got to go back to his advisors and ask them, “Hey, is this a good idea” and you never know if they’re going to go, “No, we’ve done the numbers and it seems to suggest that people will feel uncomfortable if the Governor is dabbling in acting.”
All of which we find a bit bizarre, as a) surely it’s a bit late in the day to start creating new FX sequences as important as that and b) the "Making of" companion book for Terminator 4 says that they’ve used CGI to create a Terminator that looks like the young Arnie for a punch-up at the end of the film. Maybe they just don’t want to spoil the surprise. Or it's all tied up with lawyers. Or something.
McG also has this to say about another returning character: "Ultimately, we always wanted to pepper in the role of Sarah Connor but we wanted to wait until we had a cut of the film. We wanted to dance that delicate line between honouring the first two pictures but by the same token not just getting obsessed with recreating the ideas that were established there. So you got to find that balance. I just think that it’s appropriate. If the first movie is about saving Sarah Connor and the second movie is about saving John Connor, then our movie is about saving Kyle Reese. Therefore, that’s sort of the trinity of the Connors. It just seems appropriate to have her in the picture in some capacity."