I predict that for iOS 5 Apple will announce new APIs that make it is for developers to add cloud sync (especially of documents) to their iOS apps, in competition with Dropbox. In addition, apps on the Mac App store will be able to use these APIs. It just makes sense. The best apps already do this (Calendar, Email, Contacts, Instapaper, Simplenote, Evernote, etc. etc. etc.)
Clearly the app that would best benefit most from this would be iTunes, for wireless Music and Video sync (and backup). We'll see how it turns out in probably the next year if not months.