How to find total number of nodes?

[hazzey]

I was trying to figure out how the estimate of total bitcoin nodes as shown at http://stats.bitcoin.it/rrd/nodes_total-month.png is created.  I am guessing that this comes from the number of addresses that the bitcoin client has seen.  Is there a RPC call to get this number? 

I realize that this method would be fairly inaccurate at best.  So does anyone know how I could go about making a similar graph?  Maybe I am way off in my assumption.

[genjix]

https://en.bitcoin.it/wiki/Protocol_specification#getaddr

or use IRC boostrap nodes