I actually was having this issue, and was going to post about it with a "me too..." but then decided to take some test shots and call apple.
I did this and the flash failed as it had been doing, it lights for focus but then goes dimmer and out during the shot rather than full bright - giving darker shots than if I had not used the flash at all (of course dark room shots are just black).
The attached shots shots are in a lamp lit room (lamp is just out of shot on the right of the photos).
Took up my old iphone 4 and tested it in the same lighting/room and it worked like a charm.
call went through to apple and as I talked to the poor rep to give him my serial I went for the home button to switch out of the camera app and accidentally hit the camera button and the damn thing worked fine.
Apple is keeping the call as open and filed it intermittent, and said if it crops up again to please call. Not a particularly helpful post on my part but to say the OP is not alone and its odd behavior for new equipment.
So odd.