If you know what you're doing(which Apple should since they created the constraint system), then it's not that hard especially with the current constraint structure. There really isn't any technical reason why they shouldn't be able to reimplement iTunes natively and use backend API's to just serve content.
Having played around with apples layout and general UI related stuff myself quite a lot I can tell you that it's a lot harder than webviews. We are talking perhaps 50X for the kind of stuff like the App store.
