I created a 2 layer PCB with only silkscreen on top and traces on the bottom.
In the 3D Preview the top plane of the PCB is not visible but the silkscreen is,
the bottom is black, the holes show and the traces are translucent.
The examples preview as they should so there is something amiss with my design.
-- 15 Oct 2013, 13:46 --
I found the answer last night!
Somehow the board border got deleted.
I re-did the border and all is well.