Apple - Is there any way to add an ePub book to iBooks without iTunes?
If you email the epub file to yourself, opening the attachment in iBooks will be an option in iOS Mail.
Now you can use dropbox.
Put the epub in dropbox and open them using the dropbox app in ipad.
Dropbox will not be able to open epub but you can choose to open it in ibooks or other supported apps.
GoodReader is capable of this. You can add a bookmark in Safari which will save the linked ePub into GoodReader's "filesystem", from where you can tap the ePub file and open it in iBooks.
As an aside, GoodReader makes a fairly good Swiss army knife tool for file management on non-jail broken iPads.