user238: Impossible to answer accurately, but I suppose one could come up with educated guesses and ballpark figures from the number of nodes on the network, or number of users on popular wallet services.

user78: Check out my answer to this question about geo data.
In fact, I would recommend creating one question about "How can you get statistics about Bitcoin users / clients" instead of asking about every different slicing of the data.
Is there data on the geographic distribution of bitcoin users?
http://www.reddit.com/r/bitcoin just crossed 10,000 users a few days ago (Sep 2012), to help you get some estimate.
https://bitcointalk.org/index.php?action=stats - Bitcointalk currently has 64,175 users.

user217: Due to the nature of Bitcoin there is no precise answer to this question. However, this site displays some statistics on the number of nodes in the network over time: http://bitcoinstatus.rowit.co.uk/

238: <p>Impossible to answer accurately, but I suppose one could come up with educated guesses and ballpark figures from the number of nodes on the network, or number of users on popular wallet services.</p>
 78: <p>Check out my answer to this question about geo data.
In fact, I would recommend creating one question about "How can you get statistics about Bitcoin users / clients" instead of asking about every different slicing of the data.</p>

<p><a href="https://bitcoin.stackexchange.com/questions/696/is-there-data-on-the-geographic-distribution-of-bitcoin-users/749#749">Is there data on the geographic distribution of bitcoin users?</a></p>

<p><a href="http://www.reddit.com/r/bitcoin" rel="nofollow noreferrer">http://www.reddit.com/r/bitcoin</a> just crossed 10,000 users a few days ago (Sep 2012), to help you get some estimate.</p>

<p><a href="https://bitcointalk.org/index.php?action=stats" rel="nofollow noreferrer">https://bitcointalk.org/index.php?action=stats</a> - Bitcointalk currently has 64,175 users.</p>
 217: <p>Due to the nature of Bitcoin there is no precise answer to this question. However, this site displays some statistics on the number of nodes in the network over time: <a href="http://bitcoinstatus.rowit.co.uk/">http://bitcoinstatus.rowit.co.uk/</a></p>
