The Touchdown

First note that the armband on Roethlisberger's right arm is below his elbow. Therefore, with the ball tucked into his elbow, the tip of the ball would advance slightly further than the top of the armband. Now notice that Roethlisberger's arm is over half of the goal line. Outlined in yellow is the approximate location of the ball, as it is never clearly seen. Because of this, the required conclusive evidence to overturn the call on the field is non-existent.

Furthermore, when Roethlisberger appeared on Letterman the next day and said that he didn't think he got in, he meant that at the time of the play he did not think that he had made it it. He explains this further in an interview with Jim Rome, where he says that after watching the tape, he does think that it is a touchdown.