I didn't notice any announcement about this so I'm posting this one.
Amazon now (since mid-April 2010) have available a Kindle Previewer application for Windows (XP, Vista, 7) and Mac OS X Intel (10.5, 10.6).
This allows you to view DRM-free Mobipocket files (such as are produced using Kindlegen or Mobipocket Creator, or, indeed, Calibre), and see how they'll appear on the Kindle, Kindle DX and Kindle for iPhone. Well, approximately, anyway. I suspect it will use Kindle 2 settings for the Kindle sized screen, but I haven't double-checked.
http://www.amazon.com/gp/feature.htm...cId=1000234621