We took our son to see Nemo and he will be 3 in October. This was his first time in a movie theater, so I tried to prepare him by telling him it would be dark and loud and there would be a scary shark on the "big tv screen." I don't know if this did more harm than good because he told me before hand - "I'm not going then."
oh my - LOL
Well, we convinced him that it would be just a "little bit scary" like when we say "boo" and startle daddy and he agreed to try it. We also told him we could get up and leave if he didn't like it - that seemed to calm him down more than anything - just knowing that we weren't forcing him to stay.
He sat very well for about 2/3 of the film and then got fidigity. We stuck it out though (some more popcorn and a drink helped!) and he really liked the ending.
Of course, the when it was over he said "rewind it and let's watch it again, Momma!"
I think if you go, just be prepared to get up and leave if he doesn't like it. The really scary stuff is pretty early on, and if you make it through that, he might just surprise you and stay for the whole thing!