Software as a service on iOS with subscriptions?

We are developing an app which we want t开发者_开发知识库o sell as a subscription rather than a onetime purchase on the app store. We'd like to distribute the app in a free but limited version. The pro features can be unlocked on a subscription basis. That means if you cancel your subscriptions, the pro features would be unavailable again. We're not sure if this is possible with the current apple guidelines and the in app subscription model. Has anyone got any experience with similar projects?

We're not sure either see this link

http://www.solidblogger.com/sony-reader-rejected-app-store/

http://pulse2.com/2011/02/01/apple-rejected-sonys-reader-iphone-app-because-they-dont-use-in-app-purchase-for-selling-content/

Still there are many app available on app store which provide special feature which user has to purchase outside the app store... but nobody can guarantee that....