Basically a GetMapKml call will fire up Google Earth, zoom to the region defined by the Map Definition, and that will be all she wrote. You'll see the map layers in the layers pane, but the features in those map layers do not get streamed across from MapGuide's KML service.
Unfortunately, Google Earth doesn't seem to have any useful way to diagnose Network Links and MapGuide's logs don't seem to reveal much useful information either.
I'm setting up a test matrix of various versions of Google Earth and MapGuide to see what more information I can ascertain. It would be nice to know what has changed with regards to Google Earth's KML handling since v4.3 (where this problem seems to have originated from)