So this was the big deal in the iOS release for Apple. Possibly less so for the majority of users. I think it's safe to assume that we're only at the very start of iCloud development and I'll grow somewhat in the next few years, but in a similar way to the success of the iPhone it's the apps that are going to make the difference between 'cool' and actually very useful. I didn't mention it in my initial iOS post, so here are my thoughts. At it's heard iCloud is for most of us (e-mail, calendar and contacts storage aside) actually just a central repository for app data; that is settings and documents. These documents are safely stored on the server and accessible from any device that has the app, or a compatible app installed. So take Pages: Have pages on your iPhone, iPad and Mac? Well the same documents will be available on each in more-or-less real time (allowing for upload and download times) via iCloud.
It is of course a little more than this, but for the majority of users, this is the functionality that cloud computing provides that will be of the most regular and practical use. This is the primary difference between iCloud and MobileMe, which allowed you to access remote storage as you would a disk. As part of a general move away from traditional file handling, apps are now becoming more and more responsible for their own documents and the way that they are organised.
This feature as a lot of promise and really is the tip of the iceberg in terms of iCloud's potential.