It’s disturbs me a little that everything in the Apple ecosystem, from App Stores, to App icons on iPhone home screens and on Launcher for Mac, to application specific file systems, is stepping backwards from a document centric view of computing back to an application centric view. The iCloud experience appears to confirm this direction. I’m not sure what I think about this.  
iCloud isn’t just an Apple branded Dropbox.  The difference between iCloud and Dropbox based sync is application versus document-centricity.  Earlier in the history of computing there was a push away from application based computing, where the focus was on which application you were using to the idea of using multiple micro applications to edit documents. Microsoft’s object linking and embedded was an example  of this.  The old Microsoft Binder was in a way also a step towards this idea. Most explicitly, the Apple lead consortium developing the OpenDoc initiate had this idea at its core. 
All of these products are now dead.  More recently the dream of document-centric computing itself has died.   Perhaps to be replaced by synchronisation and mobility.  
The difference between Dropbox and iCloud synchronization is that Dropbox is theoretically just a file system. Multiple applications could easily edit the same files – as long as they pointed to the same file in Dropbox.  This was also true across platforms. If you have a document that you edit on your iPad and sync with Dropbox you can edit that same file, using a different application, on your PC. 
The iCloud experience is completely different.  The only way to edit a document across platforms or devices is to use a version of the application for each device. Not a compatible application. Not a micro application that uses the same file format. But the equivalent application from the same vendor.  Usually this additional application is purchased at an additional cost. 
This is the most interesting part of the iOS5 experience. Not because it’s any easier than Dropbox based synchronization but because it may actually make me change the desktop application that I use purely based on iCloud support.  It may also make me buy one application purely for synchronisation while I might use another for specific editing on a particular platform (after manually “syncing” on the platform).