With an ArcInfo license for ArcGIS, you can use ArcToolbox to export from .shp to .kml and vice versa. But this can't be done with an ArcView license using standard tools.
However, there appear to be many scripts out there to do conversions to and from KML. These particular two have good documentation that can be followed by a lay GIS user like me. That being said, here are the URL's:
