This news seems kind of old, we already knew that his likeness was going to be used in the movie because McG said so. He will not physically be in the movie himself, there will be a 1980's version of Arnold in the film. I guess there has been a lot of back and fourth news that has been going around, but this is final confirmation. Here is the report.
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.
So there it is our final confirmation that a CGI Arnold Schwarzenegger will be in 'Terminator Salvation'.