From the ArcGIS Explorer blog
Explorer at Where 2.0
Earlier this morning here at Where 2.0 in San Francisco, John Hanke, Director of Google Earth and Maps, and Jack Dangermond, ESRI President, partnered in a presentation which showcased some of the capabilities of ArcGIS Server 9.3. ArcGIS Server was used to publish KML that was viewed in Google Earth. The KML showed the result of a fire model with predicted burn times from the current fire perimeter (red line at right).
While ArcGIS Explorer is tightly coupled with ArcGIS Server, and has been designed specifically to leverage ArcGIS Server capabilities, ESRI's open architecture also provides support for Google Earth, Google Maps, Virtual Earth, and other custom viewers.