Matt mentions it here:
http://wordpress.com/blog/2008/05/02/april-monotone/
>>>There were 475 posts made via the Atom API (234,831 with XML-RPC).
I didn't know about XML-RPC until raincoaster educated me (I now use BlogDesk for longer posts with many photos, bypassing the web method).
What is Atom and how is it used and what software uses it?