Google Maps now supports the GeoRSS file format, and the Google Maps API now allows you to add geographic data in the form of KML files to your mashups, the official Google Maps API blog reports. Frank Taylor of the unofficial Google Earth blog throws in some caveats:
The KML Google Maps can handle is still limited. It can handle image overlays, polygons, lines, and placemarks. But, only of limited size. Hundreds of placemarks will throw it off. Or large size KML files. Also, network links are not supported. But, Google has been gradually releasing more support, so hopefully these limitations will get fixed in the future.
Frank | Google Blogoscoped
Google, Google Maps, API, KML, GeoRSS