Okay, here's my take on the license agreement.
By submitting a book created with iBook Author to the iBook store for sale (if you submit a free book the rules are different), you agree to distribute that book exclusively through iBook. The license is vague, though. It fails to distinguish between the content and the format. There is enough wiggle room in it that Apple could, if so inclined, persuade a judge that the intent is to gain exclusive distribution of the content. Here is some of the vague wording:
It is certain that an iBook format book would be exclusive to distribution by Apple. It is less certain if an author could create a pdf for distribution through, for example, DriveThruRPG and create a separate iBook version for sale with Apple and still stay within the terms of the license.any book or other work you generate using this software (a “Work”)
Due to the nature of digital distribution, Apple also gains the right to copy, store and transmit the work. Otherwise they wouldn't be able to sell it for you. They do not gain any ownership of it. The only rights they claim are those necessary to make the system work.
All of that said, the lack of any option for non-exclusive distribution is obnoxious, regardless of the intent. Many self-publish distributors have a two-tier agreement: You get a higher percentage of profits if you agree to exclusivity. DriveThruRPG, for example, offers a 70% royalty if you agree to sell only through them and a 65% royalty if you want to sell through multiple distributors. iStockPhoto pays 45% for exclusive and 15% for non-exclusive distribution.
On the other hand, when it comes to eBooks, Apple knows that it has its hands full competing with Amazon (and maybe Barnes & Noble if they can get their act together in time). Allowing an author to publish to both iTunes and the Kindle means that they're bringing the iPad into more direct competition with devices that cost less than 1/6th of the iPad's sticker and have an equally versatile selection (though admittedly not nearly so much functionality). If they can manage to cordon off a portion of the market with exclusivity agreements, they might be able to retain enough of a unique offering to keep their product relevant. And if Amazon wins that particular tug-of-war, Apple can always rescind the exclusivity paragraph.