WeatherBug has a new feature via their “labs” that takes either Live Local or Google Maps and overlays their weather data on top of it. Its got some bugs (if you switch between Live Local or Google Maps the zoom doesn’t stay constant), but it does allow you to use whatever mapping product you wish. They have an API that you can use so I’m sure this would be just as easy to add to Google Earth (I’ll assume someone already has done that) or even ArcWeb Services.

Weatherbug Mashup in Google Earth and Virtual Earth