I'm not an expert on Gibsons, but am interested in Alex's gear so I did a little research. My guess is that it's actually an ES-345. At the time of Hemispheres he had a 335, 345 and 355.
Assuming this is the video you're referring to: http://www.youtube.com/watch?v=MtWxyRi8mwY
If you pause it at 0:48 seconds, you can see it has a varitone switch, which I think rules out the 335. The fretboard inlays are split and the headstock inlay is the crown, both of which are features of the 345, not 355. And as far as I've read, the only 355 he's had is the creme one he played on the latest tour.
This is all based on a little bit of searching I just did, so there may be exceptions to those features, special editions, etc. I'm sure someone more familiar with those models of Gibsons can confirm which he's playing.