Will any two applications be allowed to share data on iCloud, or will they be restricted to applications that are, by some definition, the "same application"?
One of Apple's primary examples for iCloud data sharing was the iWork suite, where a single Pages document could be accessed on the Mac, iPhone and iPad all through iCloud. Are all of these the 'same application'?
What does it mean to be the 'same application'?
In iOS the simplest answer would be the "app id" that all applications sold through the iOS app store are required to register. If you write a universal application for iOS that can be run on the iPad and iPhone, these would have the same app id, but if you sell two distinct applications, one for iPad and one for iPhone, these will have different app ids.
Interestingly, the Mac app store also requires app ids. If you sell an application with the same name and roughly the same functions through the Mac app store and iOS app stores, do you give them the same app ids?
iCloud Data Sharing Restricted by App Id
If iCloud were restricted so that you could share data through multiple applications with the same app id, this would presumably encourage developers to build universal applications rather than distinct iPhone and iPad versions if there were any value in data sharing.
For instance, if Plants vs. Zombies could only share game state data if it were a universal app, then I'd strongly prefer PopCap to start selling universal applications, even if that meant the cost of their iPhone version were to rise accordingly.
Similarly, I have OmniFocus for Mac, iPhone and iOS; these are distinct applications -- will they be allowed to share data through iCloud?
Unrestricted iCloud Data Sharing
If iCloud data sharing were allowed between any two applications who were to agree on some details (namespace, format, etc.), this could enable new kinds of application integration, which would be interesting in its own right, as well as allowing iPad and iPhone versions of the same application to share data without becoming universal apps.
I'm not trying to present a false dichotomy here -- there are probably other options, but I don't know what they are, and i'm not going to try and guess. I'm interested to see how iOS and Mac apps are allowed to share data using iCloud and what the impact of that sharing will have on universal apps and application integration.