How to get statistics about Bitcoin users?
The closest thing I'm aware of is RowIT's Bitcoin Stats Page which counts listening hosts, unique IPs and several other statistics which come closer to, but still does not directly equal, a count of users. It is, at least, less likely for one user to run a client from two IP addresses than to simply run two clients from the same address, so unique IP statistics might be somewhat more useful to you.

Realistically, Bitcoin was explicitly created with privacy in mind, so as much data mining as we are capable of, there are certain statistics you're simply unlikely to ever lay hands on.