iCloud, though the public has not seen it, has been regarded as Apple's newest weapon and its best hope to supplant the services offered by Google, Microsoft, and others. It will be what MobileMe was meant to be. And it could be Apple's final effort in trying to convince users that it knows what cloud computing is.
Even at this point, iCloud may well be nothing more than a glorified online storage for files, photos, and music. But as with everything Apple, it offers pinpoint improvements over time and adds new innovative features and apps when they feel it has been baked well enough.
And it could be possible that as iCloud evolves further, it could merge with iOS and OS X further in the coming years. Just as what we are seeing now, iOS and OS X share features and that could become more so with whatever OS comes after Lion
Google's Chrome OS is a vision that we could see future embedded devices head towards. And though that could be challenged by Windows and OS X, there is support for this path. Mozilla's new project "Boot To Gecko" is aimed at the smartphone but just as some earlier netbook makers were anxious to make Android their default OS, don't think "Boot To Gecko" will be used exclusively for mobile phones.
Obviously, this is merely my own speculation. However, a cloud-based OS where features are "downloaded" and accessed by the user would core OS be updated by Apple more easily. And then Apple can build apps around it. I reckon this could also be more efficient.
It was from reading this Yahoo post regarding iTunes getting iCloud integration that made me realize just how important iCloud could become. It won't be just the core apps like iTunes, iLife, and iWorks but Apple could extend integrations with more advanced features in future updates.
This could lead to faster boot times and smaller footprints for future iOS devices and Macs. And moreover, Apple could offload heavy lifting by its server farms as well.