Incidentally, you do know that you can buy kindle or Nook apps for your ipad? I was assuming you wanted a DE-specific app.
EDIT Actually, those apps are free--you just pay for what you buy through them, if that helps you any.
Incidentally, O'Reilly, my publisher, uses PDF for real galley purposes, although they do create epub editions of their books for distribution. You might want to ask the publishers you work with if there's another format available.