However, Apple does require that if a publisher chooses to sell a digital subscription separately outside of the app, that same subscription offer must be made available, at the same price or less, to customers who wish to subscribe from within the app.
In addition, publishers may no longer provide links in their apps (to a web site, for example) which allow the customer to purchase content or subscriptions outside of the app.
http://www.apple.com/pr/library/2011/02/15appstore.html
Also: http://news.ycombinator.com/item?id=2535773