Hi Rob,
I'm not sure if there is a driver which works directly with xelatex, but you might want to take a look at LaTeXML or TeX4ht. Both of these tools allow you to create xhtml. If you use a custom style sheet, you can even specify extremely strict compliance. Converting from XHTML to epub is pretty straightforward.
The last time I checked, TeX4ht was rather terrible at working with xelatex source code. I hadn't looked at LaTeXML, but any tool that implements its own interpretation engine for TeX source is basically going to be "not good enough".
Building an epub file from the boxes and typesetting as found in a dvi/xdv file means all the computations have already been performed (and are, presumably, correct!) and will allow the generation of an alternative format document with the same (default) layout as the dvi/xdv or a PDF generated off that dvi/xdv.
- Mike -------------------------------------------------- Subscriptions, Archive, and List information, etc.: http://tug.org/mailman/listinfo/xetex
