What's on page 47?

It is currently unknown. According to the film's FAQ on IMDb:

It is never made clear in the movie what was on page 47, and some speculate that it exists as a cliffhanger solely to set up another sequel. On the DVD commentary, director Jon Turteltaub thrice says something about the contents of page 47, but his microphone fails mysteriously all three times. He also rebuffs the sequel theory, saying that the references to page 47 exist to show how much the President began to trust Ben Gates and to establish a closeness between the characters.

However, since National Treasure 3 appears to be in production now, this could always be revealed there or at least referenced, despite Turteltaub's assertion.

It's thoroughly set-up that the content on page 47 concerns Area 51. Riley's introduction of The Book of Secrets builds towards his assertion that it contains information on Area 51. The topic of Area 51 then conspicuously high-jacks the whole dialogue. Abigail calls it an "urban legend" and Riley responds: "Is it Abigail, is it?"

Abigail: "It's just totally-"

Riley: "-Crazy?"

Abigail: "Yeah!"

Riley: "Huh, because last time I checked, we pretty much make our living on crazy"

Ben: "He's got a point."

An inordinate amount of emphasis is placed on this topic at this point in the dialogue where it is neither necessary for plot, character, nor theme. It's a tangent that is so seemingly off-colour to the National Treasure franchise that it fights to justify its own validity. This, I suggest, is to properly establish Area 51 as a very real possibility in the series.

When the group finally finds The Book of Secrets, Riley identifies the Area 51 section as Ben flips through the contents--but little time can be spared. We are given on-screen confirmation that Area 51 content in fact exists within The Book of Secrets, making it the only confirmed possibility for what could be contained on page 47.

Ben's last words on the subject give us another hint. At the very end of the film, he calls the contents of page 47 "life altering," when describing them to the President. It should be recalled at this point that Riley first calls Area 51 "the coup de grâce" which means "the death blow." These phrases are almost interchangeable, since something that is life-altering can also be said to be life-ending, since every new beginning represents an ending. Behind some wordplay, the script doubles back on itself as both Riley and Ben use the same terminology to refer both to Area 51 and the contents of page 47--implying that they are one and the same.

But why the number 47? If this was any movie but National Treasure it wouldn't be nearly as appropriate to assume that there must be some hidden meaning behind that number. We know from the combination of the Resolute desks being the year 1846 that in National Treasure, the go-to encoded meaning for a number is a year of historical significance. This was Ben's first assumption and it was correct. That fact alone is its own cipher: we are being told to think of numbers as dates. If 47 refers to 1947, then we are looking at the same year that an ET craft crash-landed in Roswell New Mexico, effectively beginning the story of Area 51. The page number 47 points to its own historical content.

If it is the case that National Treasure 3 is all about around Area 51, then we may have our answer as to why the script is taking so long. Such a departure from the traditional National Treasure mainstream historical content must be giving pause to its writers and executives. But remember that even Indiana Jones (a similar type of franchise) eventually ventured into extraterrestrial territory with panache. The facts point us there now with National Treasure 3. The sensitivity around the subject of Area 51 being a project officially classified in the alleged interest of national security also would explain why Turteltaub would want to remain noncommittal in his answers around the question, so that he can easily take the content of the sequel into safer territory if need be.

The last question remaining is why the President would require Ben's help regarding the contents of page 47 if it's about Area 51? It has recently been speculated that certain presidents have wanted to disclose the reality of the ET engagement in human affairs to the world but for various complex reasons cannot. It could be that the National Treasure president beseeches Ben, a man with a trusted record of proving historical mythology to be historical fact, for assistance on breaking the secrecy around Area 51 on his behalf. This would be the appropriately "life-altering" event, as Ben promises.