My kid decided she was going to do a paper for school on bitcoin since i talk about it so much, then tonight over dinner she asks me a question i had no definitive answer to.

How many bitcoin addresses currently hold bitcoin in them? more than a 0 balance.

After a few hours searching the WWW i have found nothing more than a few charts that show "addresses used by day"  and 10,000 pages telling me how many bitcoins have already been mined which i already know.

Is there any way to retrieve that data from the bitcoin client, or any website that accurately has that data? i need to find something that she can use to cite her source with. 

Any help would be appreciated.