The Bitcoin Map is in fact a nameless project based on the analysis of addresses exchanged by bitcoin nodes.
Based on the collected informations, and by passing the collected IPs through Maxmind's GeoIP database, a map was created.
It was suggested on IRC to produce a map similar to torservers' one, using Google's kml format.
Then other resources were made available, including the number of online nodes.
Bitcoin nodes over time
It is currently still unknown how reliable this information is. It represent the total number of unique IPs seen within the last 3 hours on the network.