I think that's probably a bit of an optical illusion because of the large opening around the mech-eye.

As for how they did it, I would imagine it'd be a combination of prosthetics and maybe some CG replacement, using a partial blue-screen mask or something.

Could be wrong, though. Compositing isn't my area of expertise.

From the trailer, it looks like T3 might actually turn out pretty decent. I was worried. But does Claire Danes' character ever speak? I don't recall her ever saying anyting in the trailer. lol

A little off topic, but speaking of trailers, don't you find it funny that for League of Extraordinary Gentlemen, only Sean Connery is named as one of the actors? Pretty small league.