“iOS 6 for Developers” says this:
In-App Hosted Content
Host In-App Purchase content on Apple’s servers, so you can easily add, manage, and deliver In-App Purchase content for your app.
Apple will host the content for in-app purchases, so developers will not have to embed their content in the app on the device. And if they were going to use their own server for content and verification this will no longer be needed since Apple will store the content.
If all the content is stored on Apple’s servers for iOS 6, how will older iOS versions be handled? Will this require a backup storage, either on the device or on a separate server?
You aren’t required to use Apple’s content hosting for your IAPs, even in iOS 6.
If your app requires iOS 6, you can just use Apple’s content hosting exclusively.
If your app runs on iOS versions older than iOS 6, you will probably find it easiest to host your content on your own server exclusively (or embed the purchasable content in your app exclusively), and not use Apple’s content hosting even if the user has iOS 6.