"Airliner would have survived xmas day bomb"

I don't know if Vulcan's right or not. 

I believe that an important point is whether the AC was still pressurized or not, and if the test accurately took into account that fact.  I've never seen data on that, but in the actual event, it must at least have been in descent, given the time frame of 20 minutes before landing.  It might still have been fully pressurized, but probably not. 

In an AC at altitude, at or near maximum cabin differential pressure, an additional pressure spike from an explosion would almost certainly cause a breech and the great volume of air bottled up in the cabin would probably have done the rest of the job.  Once the eggshell is breached, the internal pressure does the rest.  Maximum cabin differential pressure would have represented a "worst case", in my opinion.

If fully depressurized or even partly so, the mechanical stress (pre-load would be a good way to characterize it) on the AC structure from the pressurization would have been less, and the volume of bottled up air available to compound the damage would also have been less.  If the explosive force were marginal, that single factor, i.e. the cabin differential pressure at the moment of the explosion, might very well be the difference between a casualty and a catastrophe.

In any event, the BVD Bomb was a dud and succeeded only in inflicting on the jihadist a little of what he richly deserved - instead of the expected dozens of virgins he got toasted testicles. Good deal - sometimes things just work out!  He may even qualify for a Darwin Award by having removed himself from the gene pool - I hope so.

Whether his underwear ordnance would or would not have brought down the plane is pretty much academic in this case.  Far more important than whether the bomb was large enough to bring down the AC is the matter of how it ever made it through the security at two airports and got aboard in the first place.  Arguments about whether it was large enough or whether the recent test was valid are pretty much moot. 

What was the point of the test exactly - to decide if we need to build stronger AC cabins? 

There are at least three things about this "test" that bother me.

1) The aircraft involved in the real incident was an A330 and the test was done on a 747.  Different AC from different manufacturers - only very general conclusions could be drawn.

2) A ground level test with doors open is another source of disparity from the real event.  Even if internal partitions were used to isolate the section where the test explosion occurred, the bottled up test volume would have been quite different from a full A330 cabin and would certainly have affected the results.

3) If the cabin of the real aircraft were fully depressurized at 10,000 feet, resulting in a cabin differential pressure of 0, then the postulated explosion would have been in that environment, i.e. at a density altitude of about 10,000 feet.  That lower air pressure would have affected the explosive force.  If, on the other hand, the cabin was still partially pressurized, that was not simulated in the test.  In either case, the test conditions were not the same as they were in the event being simulated.

What in the world was the purpose of the test, other than a media inspired publicity stunt?  Regardless of the result, what is the value of this "stunt"?  It has no bearing on anything.

- Is it a possible defense strategy for the BVD Bomber? - "Your honor, tests have proven that the device was too small to have brought down the plane, therefore, I move to have my client's charge reduced to misdemeanor disorderly conduct."

- Should security regulations be relaxed so that people can bring explosive devices aboard as long as they aren't large enough to bring the AC down?

- Are aircraft certification requirements going to be changed to require the cabins to be able to survive the detonation of explosive devices small enough to be concealed in underwear?

- Will existing aircraft hulls have to be strengthened to withstand a certain sized blast?

Whether the test was valid or not, there's nothing important to be learned from it.