I like him too, and I think he will be great in the part of "Grown Up" John Conner; passionate about winning the war, yet also coldly detached by the knowledge that he almost prevented it TWICE... kind of like Batman-Mode, but with machine guns

What worries me in particular is that, if Arnie doesn't play the robot, who will? I honestly think that the weak point of T3 was the person they chose to play the "Terminatrix", Ms.Kristanna Loken. Now don't get me wrong - very talented actress, worked very hard in the role, but ultimately she just wasn't scary.
Arnie? Monosyllabic, hulking big guy built out of slabs of beef. Didn't take much imagination to see him as just skin stretched over a man-shaped, yet dangerously strong machine.
Robert Patrick? Very flexible in the role that required it, taking on the right tone and demeanor of his 'disguise' yet always conveying the sort of cold menace needed in an inhuman killing machine.
Kristanna Loken.... looked like a woman full of special effects and a lot of wire-work.
They NEED to bring back the 'scary' Terminators, something big and intimidating in order to portray not just character (the robot) but the super-intelligent, globe-spanning system that guides it on with a single unswerving goal: to annihilate mankind forever. Personally, I think they should bring in the 'rubber skinned' Terminators that were mentioned in the first film - something very nearly human, but at the same time
not quite right until it's too late.
That is the kind of creepy that is needed - we've already had Unstoppable Juggernaut, the Chameleon and the Puppet Master, so now we need to go right back to the beginning and bring in that which the Terminator films are about; Humanity and it's determination to survive, in the face of something that looks like it but is otherwise alien in every other way.
Well, one can dream, I guess