That's a good point, I'm not quite sure how to answer that. I know almost nothing of quantum mechanics. My logic is simply that if you had a large enough calculator and nothing was truly random that you could predict us having this conversation from the moment of the big bang. That's incompatible with true freedom of will (which someone is sure to ask me to define, I swear metaphysics suffers from death by definitions). But given true randomness:
1) Person faced with ethical decision
2) Random events somewhere in the brain
3) _____________
4) Unpredictable autonomous decision!
Or something like that.
Edit: moyix's is a much better response than mine