Apple sent us an e-mail today with details on how someone could sign up to sell their own books in the iBookstore. Their books would have to adhere to these criteria: each one would need to have a 13-digit ISBN, be in ePub format, validate against epubcheck 1.0.5, and contain no unmanifested files. If you are not familiar with any of these Apple suggests working with their aggregation partners.
In addition to these requirements you will need to have a US Tax ID, a valid iTunes Store account with a credit card on file, and an Intel-based Mac running OS X 10.5 or later.
Looks like you need an Apple Connect account to get info.