I can't speak for the others with this problem, but in my case...here is what happens:
E-brake has been set overnight. Clutch in, start the engine, shift to neutral, clutch out. Let the engine warm up. Clutch in, shift into 1st, release e-brake, let clutch out. POP!
The sound never occurs in the course of normal driving - only when the car has been sitting for an extended period of time - say more than 6-8 hours or so - and when the e-brake has been set, and when it does happen, it is a singular occurrence only. The problem is especially evident when it has either been raining or has been very damp out while the car has been sitting.
I like the theory of a bit of oxidation "welding" the pads to the rotors; I'm surprised that more others haven't experienced the same problem, though.
