The opening weekend of any 'blockbuster' movie is really just a barometer for how good the hype was, how good the trailer is, and how much pent up demand there was for the adaptation. This is true for X-Men, X-Files, Watchmen, Batman, and our beloved crew of the Enterprise.
That second week, and the subsequent weeks, is very dependent on the reviews. These are the people who waited for someone else to go see it opening weekend, and then wait to hear what they said about the movie.
Star Trek is getting great reviews, and not just from the newspaper shills-- audiences generally like the film. This is different than the (lack of) buzz about Wolverine, and the outright confusion about the Watchmen. It's more along the lines of Batman Begins: your older sister asked you "Really? Another Batman movie?" to which you've replied "oh yeah-- it's that good."
Expect a strong 4 week run on Star Trek.