Apple has finally made their new in-app subscription policy official, and it works out well for Apple:
Subscriptions for iOS apps will work much the same way as we’ve already seen with The Daily. Publishers can charge subscriptions on a weekly, monthly, bi-monthly, quarterly, semiannually, or yearly basis, and customers can sign up via in-app purchases that get automatically billed and renewed on their iTunes accounts. With this method, Apple takes 30 percent of subscription fees.
Publishers are also free to make their subscriptions available outside of their apps, say, via their own Websites and to existing subscribers of a print edition. If customers sign up that way, publishers get to keep 100 percent of their subscription fees. However, publishers are not allowed to provide in-app links to let customers sign up outside their apps, and Apple stipulates that in-app subscription prices remain either the same or less than their counterpart offers.
Amazon has yet to comment about the future of the Kindle app on iOS devices, but it seems doubtful that they'll just hand over 30% of all their e-book sales to Apple.