Is there a way to estimate total number of bitcoin users/wallets? 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?
The number of wallets is impossible to count, as they can contain zero to thousands of addresses each (zero, as there may be users who just want to observe, or clients that only create addresses when explicitly asked to).
Counting users by number of client downloads won't give you a good estimation either, with all the web wallets that are out there, plus of course the other clients that are out there, the alternative ways of downloading the client and the double downloads.
Counting connected clients will also give bad results, again because of the web wallets, plus people who don't broadcast their IP address or simply don't start the software unless they really need to.


You may be able to estimate the total user count by asking all web wallets for their user counts, and then adding the amount of nodes on the network over a relatively long period. But then you'd still need to compensate for those who don't broadcast their address, dynamic IP addresses, etc.

So yeah, it's hard to estimate the user count.