Shri and PorkChop both raise good points, so while I would probably take the chance and buy the one-way ticket, my advice may not be the best for the original poster.
Plus, this same topic is under
discussion at FlyerTalk. This thread doesn't really clear anything up -- anecdotal evidence suggests that, well, you may or may not be required to show a return ticket.
If your credit card can swing it, buying a fully-refundable ticket (and then refunding it once you're in Hong Kong) might be the safest solution.