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.
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. :-)