A few times in the past I used a Plugin for Word2007 from aspose to save a doc as epub.
It didn't work that well.
I just saw that Aspose now has a free standalone utility to do the same thing and perhaps better. It supports also the docx format.
It is called Aspose.Words Express and here is the link.
http://www.aspose.com/community/file...s/default.aspx
I just gave it a quick test and it seems to work rather good and very fast. It is easily editable in Sigil.
I hope more people give it a try