I was at SDCC today, and saw footage of Godzilla as well as the footage of him in the Godzilla Encounter. This model is inside the Experience and I am 100% sure that this is the big G.
The Experience start with you going through a small Japanese/Tokyo downtown area, with shops and displays of Godzilla memorabilia, most of which we have already seen from other websites articles. Then you are taken into a War/Control room area where you "assist" in tracking down the big G.
Then alarms sound and you are taken into an elevator that simulates you going to the 25th floor. You get out and are moved into a room with a large glass window (screen), and Godzilla then walks in front of the building, YOU GET A FULL PROFILE OF HIM FROM THE SHOULDERS UP!! You get a good look! He then comes back and looks directly into the window, before walking away. SKREEEOOONK! Sounds exactly like he should and looks EXTREMELY menacing!
Below is the model that I am sure is what we will see in the movie, or at least an extremely close version.
Below is a possible explanation of his atomic breath. I cant read Japanese, but it looks like it is "stored" in his tail.
I am not sure what design this is, but it wasn't what I saw.
Hope you enjoy!!!