Planes are pretty much bomb proof from anything that could be carried on a person.
Screening of luggage for explosives makes sense.
Uhm... Eh, not really. In theory, no, with expertly crafted bombs and properly placement, you could easily take down an aircraft with explosives handy carried.
In practical terms, pretty much. Suicide bombers are not usually selected for their intelligence, training and expertise. They tend to be useful fools, which limits the complexity of their accomplishments. The most deadly terrorist operation conducted by Islamic militants was 2,996. Any halfway competent person could kill a lot more than that at any sporting event or political rally.
And if you can find a dozen or so people to agree to 1-lb C4 enemas and are all willing to get on the same plane in order to take it out, AND can get detonators through security, then you're also skewing the issue there as well. And I'd really like to see you defend mandatory cavity searches for everyone getting on a plane, or even random cavity searches - how do you imagine THAT will go over?
Remember, the single "butt-bomber" who managed to get a detonation did not in fact actually kill ANYONE, IIRC - not even his target right in front of him. Could such a person DAMAGE an aircraft? Sure. I imagine his seat would be messed up, and he might apparently cause some injuries, even. Doesn't seem like he'd blow even a small hole in the hull, though.
Internal bombs have several problems. They have to be installed by a skilled doctor, and would be difficult to construct in a lethal manner. Explosives themselves are mundane unless: specifically shaped or tuned, lots of it is used, or used with shrapnel.
Shaped charge or "tuned":
Shaped charge inside the body would not be very effective. The human body doesn't have tons of spare room to do nothing. Fat deposits or gastrointestinal would be my choice of placement, and that's tricky. Excellent placement would also be problematic because you'd be working with very narrow performance limits.
Lots of it being used:
Lots of it being used is unlikely unless the person is morbidly obese. Again, most folks don't have lots of internal volume for this sort of thing.
Shrapnel:
Ouch. Possible, but you'd need a very high order explosive. Because the body would act as tamping and drastically reduce both the acceleration curve as well as the ultimate velocity reached by the projectiles before they reached the necessary parts of the aircraft.
Short version long, butt bombing is not a really valid terrorist tactic unless you wanted shock value for the cleanup crew. And that it would justify the TSA using even more sexually invasive procedures.
