Remember life before cellphones? It's hard, because they are so ubiquitous and essential. iCloud marks a similar turning point, a transition from an old way of doing things. It makes computers easy for everyone, by making computing disappear.
Of everything Apple is rolling out today—and its rolling out a Hell of a lot of stuff—there is no bigger deal than iCloud. Forget Siri or Find my Phone or Notifications or any of the other numerous updates and enhancements that iOS 5 brings to the party. None of it matters compared to iCloud. For some of us, iCloud means we're never buying another computer, and for the rest of us, iCloud will be the end of computing as we have always known it.