i still think the terms are being confused because panterarosa claims that there should be no trunk flexion/back rounding in a full squat, and claims the spine should be neutral. technically, spinal flexion and a "neutral" spine are not the same thing.
now, hearing you guys say that trunk flexion has no role in full squatting makes me think that you A) have never done them, B) have done them but haven't consciously noticed trunk flexion (since you're trying to maintain a "neutral" spine), or C) haven't ever even thought about it because lots of "gurus" say the trunk should never be under flexion during a full squat (actually, they tend to use the word "neutral" but it seems that word has been used interchangeable with the thought "fully extended").
personally, my full squats are two inches shy of ass touching floor. i recently just finished doing a three week cycle of full squats, 3 second pause in the hole, taking 3 reps to near failure. and i can assure you, my trunk was under flexion in the hole, and my good form was quite enviable. if it wasn't under flexion then i would've fallen backwards (actually, it's a good thing that the trunk flexes once you reach a certain depth because if it didn't it would be really fecking hard, if not impossible, to dump the bar behind you).
if you dont believe me then try this experiment: grab a broomstick, stand beside a mirror (like a profile), get into full squat stance, hold broom in overhead squat position; now slowly descend into a full squat while keeping your trunk completely extended. if you do this right, you'll find out that you fell backwards before you even reach parallel. for me this happens about 3-4 inches shy of PLers' measurement of parallel. depending on your leverages it may vary.
still dont believe me? try this: get in the same position as before, but squat down to the deepest depth you can. you should notice that your back is straight, but because of the postion of your hips it is, technically, flexed. now, while sitting in the hole and without moving any other part of your body, slowly extend your back. if you do this right you'll find that you fell backwards quite a ways before you were able to completely extend your back.
still dont believe me? well then, i'll reference, the one and only, Pyrros Dimas...