What if that underwear bomb nestled between that loathsome terrorist's legs would've exploded? According to the BBC, it would not have taken down a Boeing 747. Notice the remarkable flexibility of the aircraft frame, absorbing the impact without losing its integrity. From this video, it appears that the airplane would still have been able to land safely.
However, the passengers within might not fare so well. The plane in this test wasn't sealed up and pressurized. Although the Christmas-bombed plane was at a low altitude at the time of the incident, those poor passengers sitting inside the sealed-up tin can might have suffered greatly from the sudden surge in pressure. We're thinking broken eardrums, at least. Perhaps they could have survived if they were wearing a Hurt Locker bomb suit.
Too bad the daily theater that is airport security wasn't impermeable enough to stop this miscreant. And what are they doing differently now? What did we learn from this? Nothing. Just keep cooperating and being afraid, and the terrorists win.