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.