From iBooks Author, you can export in iBooks, PDF, or text format. The iBooks format is pretty much a well organized version of the epub format. I just exported one, changed the extension to .epub, and loaded it into an ebook reader.
It's worthwhile to note that PDF export only outputs in a fairly useless landscape, two textbook pages per pdf page form. I have to think that this is done in this horrible way by design. Similar for your .epub "hack", I'm sure that will not be supported by Apple, ever.
On the other hand, the textbook makers currently have such a lock on school districts that I don't feel really bad about this. iPads will only get cheaper.. I'm almost positive kids will complain when they get the shitty scratched up hand-me-down iPad2 the school district has had for 5 years when iPad7 is available -- but I've had worse experiences with paper textbooks. :-)
