I completely agree with David that we're on the verge of a major revolution in software development:
For the first time since the iOS arrived in our lives, using the same app on multiple platforms comes with an added benefit, data bliss. When users look at apps for their Mac or iOS devices, they are going to actively seek those with support on other platforms. Automatic data-syncing is a huge benefit and multi-platform is going to be a big deal for enlightened Mac and iOS developers.</p>
Dropbox has been fantastic, but I think iCloud promises even more. Before sync became such a reliable reality, computing was basically a two-dimensional concept: you chose hardware, and then you chose the software to run on your hardware (not necessarily in that order).
The "cloud," for lack of a less abused term, isn't hardware or software from the end user's perspective. It's a whole new dimension that exists between hardware and software.
Perhaps the sneakiest form of iOSification that we'll soon see is the way that iCloud will, in a way, make desktop applications part web browser like their iOS counterparts. Perceptive people like John Gruber have already argued that native iOS apps are essentially web apps themselves: Many iOS apps wouldn't be what they are without a behind-the-scenes web component.
I think iCloud will make desktop apps more web-app-like without forcing developers to develop within the constraints of web browsers. This is good news, and I'm pretty excited about it.