I just think a lot of you are overanalyzing this stuff... Bringing in stuff like "buzz scores" and what-not only complicates things...
I basically look at a few simple things when a movie comes out:
How many theaters it is showing in.
What movies came out in the past that are very similar in genre, month and competition.
The PSA of those past movies.
Based on just those 3 things, you can have a pretty good idea how well a movie is likely to do... Then you can always throw in a few other attributes to shift one way or the other -- if the movie has any sellouts yet, how many people have been calling moviefone, how many ads you've seen for it, if the trailers look any good, early reviews, etc...
T3 trailers looked terrible in most people's opinion -- that right there had me hitting up the UNDER for the weekend.
League's trailers didn't look half bad, it was a comic book movie, and even a movie like The Avengers did $11 million in the same season with tougher competition (Saving Private Ryan).
Matrix 2 was front-loaded with real fans on Wed/Thurs... Just little stuff like that, most people miss -- but it had a great impact on the weekend totals for the movie.
As for Pirates... I thought the trailers didn't look all that great, but it was a Disney movie, rated so the kids could see it, lots of screens, and it was getting great early reviews... A movie like this also wouldn't be as front-loaded as a Matrix 2 or an X2...