I was looking at Bitcoin Wiki. It estimated 60,000 users (Sep 2011) based on counting the number of bitcoin clients connected to the network in the last 24 hours.
Another possible estimate could come from Sourceforge download numbers. But that include multiple download by the same person and across different version, and also exclude users that may have downloaded the client from other mirrors.
Have there been any new estimates on the total number of bitcoin users, or if this is too hard to count -- the number of unique wallets?