I also think that due to the unfair advantage of having 3&6am showings that not other film had it was destined to set records. Spidey3 and PotC didn't have those showings and PotC was just as long a film.
It's definitely not the first time a movie has had 3&6 am showings. Phantom Menace definitely did, and I think Revenge of the Sith did too. Anyway, I doubt those early morning showings accounted for *that* large a percentage of the opening day gross, as there were plenty of theaters with the movie that didn't have such showings. And most people going to 3/6am showings were going to find a theater where they could watch it opening day somewhere anyway.
I'd estimate there were approximately 75-120 people (more towards the latter) and there were two screens showing at this particular theater. I bought our tickets an hour before showtime and the auditorium was opened 20 minutes before the start of the film due to the amount of cleanup from the previous show's audience.