Mobile Documents is the same folder that contains data for apps that already work with iCloud, such as Instacast, iWork, or Galaxy on Fire 2. Sure enough, the Mobile Documents folder is the directory iCloud uses for Documents & Data, a feature available both on iOS 5 and Lion. Hidden into Lion’s ~/Library (which can be displayed in a variety of ways as we detailed in our Lion review) there’s a Mobile Documents folder that’s capable of syncing files across Macs configured with iCloud, Mac OS X Hints reports.
