And google brings up this thread after a few minutes..
Have done some tinkering myself. Used the latest kindlegen, and a cover still wouldn't show up. So I guess as far as the current "standard" goes, its fair to speak of a bug in processing (on the Kindles side).
The part that made me curious is, that older mobi's, also converted with calibre would have their cover displayed.
But its more important to play to the standard I guess.
If you could link to a more thorough description of what actually is going on, people would find it here googleing. The described workarround also isn't working anymore, there is value in documenting this as well.