Autodesk introduced a new labs tool in March for exporting autocad objects in a KML/KMZ format for Google Earth. The tool works farely well, but I have definitely had some trouble with surface exporting. It is not easy because if your surface elevation is below existing ground in GE, then it will not show up. So with some tinkering, I was able to get one of my surfaces to show up when I offset the elevation about 40' in relation to existing ground.
If you are curious as to what my stream surfaces look like, then you can download my recent tinn surface in kmz here:
http://sites.google.com/a/streamrestoration.info/civil3d/gis/STREAM.kmz?attredirects=0
You can download the Autodesk Labs Google Earth Extension here:
http://labs.autodesk.com/utilities/google_earth_extension_beta/
The command is "publishkml".
