g1smd - 7:00 am on Jul 25, 2012 (gmt 0)
It appears to be every URL crawled whether valid or not. It includes all status codes returned.
I see a huge 'ever crawled' number on a site that previously had 'infinite duplicate content', but now that site shows just a few thousand URLs indexed. Interestingly that figure is approx 10x the number that appears for the site: search.