The attempted bombing of Flight 253 last Christmas was thwarted by passengers, thankfully. But what if he had been able to get the bomb detonated? Not what you'd expect, actually.
The BBC did a test on an old decommissioned 747, putting a dummy with the same type and amount of explosives found on the underwear bomber. As you can see in the above video, the flexibility of the outside of the frame allowed it to absorb the blast. If the bomb had gone off, the plane would have still been able to fly and land.