One of my favorite features of Mountain Lion to demonstrate so far has been the iCloud Document Library. This is a way to store your iWork, TextEdit, and Preview documents in iCloud so that they are immediately accessible from other Macs on the same iCloud account as well as on connected iOS devices.
TUAW blogger and developer Erica Sadun told me the secret behind this on Wednesday. If you go to your Library folder and open the "Mobile Documents" folder, you'll notice that the name of the folder changes to iCloud (see screenshot below). That's where all of those documents are saved, and it's even possible to just drag items into the folder to add them to your iCloud Document Library.
In compatible apps, you'll find that selecting "Open" from the File menu displays a new Finder Open dialog with buttons for iCloud and "On My Mac". Selecting iCloud displays a very iOS-like dialog showing all compatible documents. Drag one document onto another, and you can create a folder -- another iOS feature. Likewise, selecting "Save As" from the File menu gives you the choice of saving a file to your iCloud Document Library.
[Source: TAUW - Click to read the full story]