The first one I think I ever fell asleep during was Star Wars Episode 1. It was a long day, we had my brothers with us and we saw it opening weekend. The theatre was packed and we were sitting up front. In order to watch it I had to look up and I already had a headache going in. On top of that the volume was so ridiculously loud that I think falling asleep was a coping mechanism for me. We went back the following week and watched it in our local theatre (saw it out of town the first time) just DH and I and I didn't fall asleep that time.
Since then, well, I find that I doze off more often than not, like Darcy, a good movie is rated on how little I slept. Its funny because I don't fall asleep at the Drive-In when sleeping conditions would be much more comfortable (sleeping bag, pillows, etc.).
When we watch movies at home, I fall asleep more often than not. DH just stops it when he sees me sleeping and then we go back to it later. I try not to watch movies for this reason.