The wikifier takes care of that:
http://trac.tiddlywiki.org/browser/Trunk/core/js/Wikifier.js
http://www.tiddlywiki.org/wiki/Dev:Wikifier
It's a fairly complex process, obviously, and should better be discussed
on the developers' group:
http://groups.google.com/group/TiddlyWikiDev/
> (and back again?)
The TiddlyWiki core doesn't need HTML-to-wikitext functionality.
I don't think there's an adaptor for this either.
-- F.