A new application called GeoFeeder by BRIGHTiSolutions allows you to convert vector data (points, lines and polygons) to GeoRSS XML.* GeoRSS is an emerging standard for encoding location as part of an RSS feed. GeoRSS feeds are designed to be consumed by geographic software such as map generators. What this means to you is that GeoFeeder will allow you to import data from ESRI, Autodesk, GML, and KML file formats into Virtual Earth.
There is a simple video on the BRIGHTiSolutions web site that demonstrates pretty clearly (despite a train passing in the background towards the end) how the app works. Microsoft's Steve Lombardi reports in his Virual Earth/Live Maps blog that the app worked as advertised for simpler applications but choked on some larger/more complex files when loading the source file.
The company is offering a 30-day trial download ... I say give it a try.