Use the Layer Preview in the Geoserver web interface to find a raster layer (for instance nurc:Arc_Sample-Global annual rainfall)
right-click on the KML link and copy link.
Go To GE 6b, add network link to My Places.
Enter a name and paste the network link, finalize.
It was reported as fixed in http://jira.codehaus.org/browse/GEOS-4345 , but with raster overlays GE show a server failure (big red X)
ok with v188.8.131.528