This issue came up a few times last year (and I think we talked about it once back then). Apparently, the rule makes that an incompletion. Likewise, apparently the rules makers are dumb-asses.
Rule 26(b)(2)(i): If the receiver catches the ball in the end zone, establishes clear position there of, takes 14 steps in bounds in the end zone, but trips over the paint at some point within the next 3 minutes and goes to the ground: he must maintain complete control of the ball while he is falling, after he hits the ground, and until he is able to get back up and demonstrate with a wave to the fans that he has in fact completed the process of securing the ball, or else the completion shall be deemed an incompletion. Once the receiver has waved to the fans, he may relinquish control of the ball without having his touchdown taken away.
Rule 26(b)(2)(ii): If a touchdown is taken away from a team pursuant to rule 26(b)(2)(i), and it happens that an extra point pursuant to that touchdown was successfully converted in the intervening time and before the receiver carelessly relinquished control of the ball, then said extra point shall also be negated.