This was implemented in calibre 0.8.11. If the opf file for the ePub has a "start" reference in the guide section. It will be used as the point that the resulting mobi first opens.
<reference type="start" title="Start here" href="../path/to/desired_start.xhtml#start" />
<reference type="toc" title="Table of Contents" href="../path/to/book.xhtml#toc" />
Just use the Tweak ePub utility to verify/add the start reference before converting.