I have my iPad now with iOS 5.1. In the email I have a .epub but when iPad's email program download the file, there is no way to open it on iBooks? Tapping or double tapping on the attachment downloaded will have no reaction. Can this be solved if I don't have a PC to place that file into Dropbox to open in iBooks?
In all cases I have tested, iOS recognizes epub files and will offer to send them directly to iBooks.
I have tested this from the email all, dropbox app as well as mobile safari.
Other apps like Evernote and AirSharing also are able to receive my epub files, so if you are not able to do this, perhaps it's an encoding issue or something about how the files are being attached to the emails and not necessarily a problem with iOS.