I don't know what the Mantis teams plans are for this, but I wrote a simple/rough plugin to do this that uses ajax to convert the markup in a text area. Only tested this in a browser. Need some feedback.
https://github.com/mludlum/MarkdownPreview

Gonna look forward for your plugin, since this would be a very useful feature, as I just posted a note on this bugtracker including a short code (and so, I embeded it in <code> tags), but without preview nor your-own-notes-edition rights, it turned out wrong...

Ie, if tipping

<code>$g_bottom_include_page = '...';</code>

Returned the plain text in the preview, then I would have removed the <code> tag myself.