why is an iCloud API a game changer? if this were a universal API that was earmarked as the standard for Cloud computing sure it would be a game changer. but all this is (and i will use the documents as an example) is a way for a dev to integrate iCloud access into an app. which i believe some apps already are able to do with dropbox and boxnet, etc. so nothing new there.
The fact that it will be pretty much built in to all iPhones means that you can design an app that uses it and you'll get FAR more people to use that feature in your app than if you have to tell them to go sign up for a Dropbox account.
I love my dropbox account, but this isn't about me...this is about devs being able to tell people "it just works!" vs. "go sign up with this other company!"
That's a big difference if you're trying to set up a new product.