iCloud looks like it will be an incredible technology for moving app data between devices. This is inherently a good thing, and it will open avenues for many new types of apps. But, there is a fundamental problem. Right now, the only way to access it is through Objective-C APIs embedded into iOS and Mac OS X. Under the hood, they are obviously talking to the network and doing the business of syncing data, but that networking layer is not exposed or documented, and would have to be reverse-engineered in order to understand and use. So the only way for developers to move their data through this system is through a pre-compiled bundle that gets referenced within an application.
This has a few interesting practical repercussions. If you build an application targeting iCloud, you can only ever put it on two platforms - Mac and iOS. You will never be able to port it to Android, WebOS, Windows Phone, or the web (mobile or desktop). If you sync data through iCloud, And, you will never be able to have a server component that can do things with your data all the time.
Here's some examples of what I'm talking about. In my To Do list app, Todolicious, one thing I would love to be able to do is to push badges to your iPhone and Mac showing the number of To Dos you have left. When you tap a To Do to mark it as done, suddenly all your devices would show the correct number on the icon. With the sync server I was building, this was fairly trivial; wait for the user's list to change, and send a signal to push the count everywhere. But if I back Todolicious with iCloud, I have no way of speaking between my server and iCloud (and I'd still need a server of some kind to send the notifications, after all).
Similarly, if I were to build a web app version of Todolicious (which I was planning on), I could not get access to that data within iCloud at all. I'd have to have either to sync to both iCloud and a custom solution (unwieldy, poor UX and network traffic, and otherwise gross), or not load existing data at all (completely negating the benefit of having such a web app).
So there is a serious ecosystem lock-in problem for apps that wish to target iCloud. All of these problems go away when iCloud is made available as a server-to-server API. A big benefit in the promise of cloud computing includes service interoperability, but right now iCloud is merely a data silo. I have filed this as a bug, rdar://9598555, for a server-to-server API (through which you could build code that speaks to iCloud on your server or on other platforms). I dearly hope Apple addresses it.
Such a server-to-server API would drastically decrease the friction of setting up cloud services to complement an iCloud-backed app, and would lead to better apps and more pleased users.