Mac OS X Hints has discovered that Macs running OS X Lion and registered with iCloud have a hidden "drop box" in the user's Library folder that allows for easy document and file syncing between Macs. A folder within ~/Library (which Lion hides by default) called "Mobile Documents" contains iWork documents synced with iOS devices via iCloud (something our own Erica Sadun discovered quite a while ago.)
"What is of use is that any files put into the ~/Library/Mobile Documents folder will automatically upload to iCloud and push to any other Mac you have that is signed in to the same iCloud account and has the 'Document & Data' iCloud preference checked," says Mac OS X Hints member CHM. "Lion even notifies you of version conflicts and allows you to resolve them when you open the document."
This functionality is broadly similar to the third-party file syncing service Dropbox, but having the service buried within a hidden user folder makes it far less useful. Files manually added to the Mobile Documents folder also apparently don't sync to iOS devices, which is another feature Dropbox does provide via its iOS app.
What's interesting about this hidden folder is that it shows that iCloud and OS X Lion have already laid the groundwork for an easy-to-use and official method for syncing files between Macs (and possibly iOS devices). This discovery makes it seem as though a very simple software update could enable the current Mac versions of iWork's applications to sync with iCloud quite easily.
iCloud is still a relatively new service, having been in public release for less than a month as of this writing. Hidden features like the Mobile Documents folder seem to indicate that Apple has broader aspirations in mind than the relatively simplistic iOS-only, iWork-only document syncing the service supports thus far.