One thing we know for sure is that there were around 60,000 people trading on Mt.Gox. No wait - 60,000 *accounts* on Mt.Gox. So, my guess is "anything between 50,000 and 100,000". But that's kind of a wild guess.

Would be nice to have a way to really track the number but that's probably hard given the peer-to-peer-nature of Bitcoin.