Here's what I have observed on my 10.8.3 system regards my questions about storing documents in iCloud from my Mac.
1. Talked to Apple Support and they told me that Apple does not backup iCloud documents. Also said documents are not encrypted in iCloud.
2. I have found that there is a local cache of application associated documents kept in OS X on my Mac in ~/Library/Mobile Documents/. Each application has a folder named like com~apple~<application name>; so Numbers documents are in ~/Library/Mobile Documents/com~apple~Numbers/Documents.
The ~/Library/Mobile Documents/ is backed up by Time Machine. So if Apple is indeed not backing up iCloud documents, the cached copy on my Mac and the Time Machine backup and my Crashplan cloud backup give me redundancy.
3. If I turn off Wi-Fi on my Mac and open Numbers I can open the files I put in iCloud and make changes. When I reestablish Wi-Fi networking the changed file gets synced with iCloud. So that works pretty much like Dropbox does.
I had some issues with one Numbers document not opening on my iPad (iOS 6). I think there is some data in it that is the problem in the file as all my other larger and smaller Numbers documents open just fine on the iPad. All were either created on the Mac or were converted from Excel using Numbers on the Mac.
User documentation about iCloud operations is scant. I'll see what is available to Developers.
