I've been trying to get the .epub working for a long time, but its not easy to convert all the equations and make them look nice. Recently I found some very good new tools[1], so hopefully I'll add .epub/.mobi to the eBook bundle soon.
Do you know of any math books that are available as .epub? I'd like to see how they implement equations... PM me if you would like to be a beta tester.
Do you plan to release an ePub or mobi-format book? I'd prefer to read it on my ereader, and PDFs don't reflow on smaller screens.