I would encourage you all to check out the DAISY Pipeline Tool:
http://code.google.com/p/daisy-pipeline/
It has a HTML to EPUB 3 converter accessible via command line or a web service API. I tested converting a Google Doc with Remix, exporting the HTML in CNX and converting to EPUB 3. It partially worked in that it generated an unpacked EPUB that I could open with the Chrome Readium App. It failed to handle MathML correctly and created the EPUB archive. I filed the following bug and hope that the main developer can fix it prior to the Accessible OER Sprint on 5/20. I will email him.
http://code.google.com/p/daisy-pipeline/issues/detail?id=310
I would encourage this team to try it out and file bugs.
Benetech uses it for DTBook/DAISY to EPUB 3 conversion in production scale.
Gerardo
Gerardo Capiel
VP Engineering, Benetech