Merrick here...

Since the beginning of production, we've been hearing many rumors regarding a possible appearance by Schwarzenegger in TERMINATOR SALVATION.

McG's been hot/cold/coy, Arnold's seemed detached and non-committal, finally offering THIS explanation as to the nature and status of his potential appearance via Twitter. It involves using CGI to bring '80s era Arnold into the film, without his having to commit to shooting new material.

And at the time of said Twitter, results were still uncertain/inconclusive.

I think that within the next week or so, we will see if they are technologically able to do that.

...said Arnold.

Variety now has all the answers for us, via a very brief article which explains McG & Arnold's unwillingness to comment too conclusively on the matter until now. In short - no one knew where or not adding a CGI Arnold into the movie could be pulled off convincingly, and only recently has Schwarzenegger given final consent for his likeness to be used. Meaning: he will be in the film.

Turns out Schwarzenegger has been secretly working with helmer McG and the effects team to reprise his signature role ... without lifting a finger.

A body-cast mold of Schwarzenegger, created when he first appeared as the muscle-ripped cyborg, provided the basis for a digital-effects version of his famous character. The figure appears in "Terminator Salvation" as a living, breathing actor.

Schwarzenegger viewed the resulting footage and gave his go-ahead just in time for McG to include the footage before the helmer completes his cut of the movie. Warners first screens the pic in early May and opens it May 21.

...says THIS write-up in Variety.