I've read a little about KML files. They are XML files that can be opened in Google Earth and Google Maps to display all kinds of data on the map. I have created a Google Maps application that displays thousands of dots representing thousands of IP addresses. The dots are red, yellow and green, depending on if they're offline, unstable or online respectively. The dots are also refreshed once per 5 seconds.
I was intrigued by the mention of KML files, seeing as they can be opened in Google Maps and Google Earth both. I have a few worries that I couldn't answer by Googling though:
You need to read about NetworkLink on NetworkLink This element can load KML(KMZ) from url.