iCloud was announced during the keynote at Apple's developer conference as the successor to MobileMe. Although part of iOS 5 the iCloud Storage APIs are already available in beta.
As far as developers are concerned iCloud is in fact much more than a MobileMe replacement. Its key feature is that it will store and sync music, photos, apps, calendars and documents to a variety of devices both mobile and desktop which are not limited to Apple's products. It can already be set up for Windows as well is iOS and OS X Lion.
iCloud Storage APIs enable apps to store documents and key value data such as application state or settings in iCloud. This enables iCloud to push documents to a user's device wirelessly and automatically and update them on any device.
In other words, the iCloud service will handle all the details of syncing, backing up and restoration including version control, conflict resolution, change notifications and security.
Given how hard it is to implement these facilities from scratch it looks worth using.
The benefits of such seamless and automatic behavior is that users are likely to install more apps, keep more apps and continue to use more apps.
Members of the iOS and Mac Developer Program can now sign up for an iCloud account and the access the new APIs and prepare their apps for the iCloud service.