The Geobloggers blog has uncovered a "hidden jewel" in Google Maps:
Adding a "Network Link" allows you to fetch KML data from remote servers. It does this in two ways, Time Based or Location Based. So *anyone* can add dynamic data to Google Maps. So if everyone currently doing Mapping Hacks fed out KML data based on the Bounding Box, you'd be able to see Craig's List Houses, Yahoo Traffic Conditions, Sexual Offenders, Metafilter Members and Flickr Photos all on the same map, as you moved around it. Add in Good Food Guides, GPS equipped feral robot dogs, geotagged web cams, (time based) meet-ups and so on and that's a pretty rich mix of public soup data.
Check out this blog post for the explanation of how to do this. There are even a few examples that geoblogger has shown.