I figured out what the problem was for my QC-15 Headphones not powering up. Its definitly a design defect with this model. The problem is that the wire that runs from the battery compartment to the negative post on the door/hinge has a fatigue failure problem. Through many repetitions of opening and closing the door to change the battery you inadvertently cause the wire to snap, very similar to how a paper clip breaks when you bend it back and forth a bunch of times. The wire is pretty hard to see too because they have it pretty well hidden on the little hinged cover and it dissappears into the headphones maybe 3/4" down from the edge of the hinge. You really have to look to see it.
What really gets my goat here is that it seems that quite a few people are experiencing an issue where the unit just fails to power up all of a sudden and end up sending the unit back to Bose for close to $100 for a "replacement". That is just plain old robbery for an almost built in failure point on a pretty expensive pair of headphones. When they get your headphones in they replace one tiny wire and then ship them to the next customer for another hundred dollars. Its insane.
The only reason I found it was that it happened right after i changed the battery which got me thinking about what could have possibly happened just from changing the battery. SO check the negative post wire to see if its broken. Also its worth mentioning that the wire could be intact and still broken internally. Mine was just snapped right in half though.