Not yet he isn't, he hasn't been cast officially. That is just a rumour.
Though he did play 70's Arnold in the movie See Arnold Run!
However, as a movie, its better than T3 but certainly third in the movie franchise so far. Its pretty damn poor. The only stand out performances for me were Sam Worthington, who virtually carries this film on his own, and Bryce Dallas Howard, who I'm biased towarRAB as I think shes gorgeous.
You have to give McG and co credit though, for getting away from the Terminator chase formula.
Anyone got the number for the whacky bus?
You can't compare the Godfather of action films with the Benny Hill of them.
Isn't the point of Terminator that by John sending the his father (ouroboros anyone?)/Terminator back, he alters the course of the future... OK, so we've learned that Skynet was an inevitability, and Judgment Day also unavoidable, but they moved in from 1997 in T2, to 2004 in T3. If you go via the SCC, its 2011.
Of course the aesthetic will change if the circumstances do.