ArsTechnica has the scoop on a new feature Microsoft recently added into builds of Windows Vista. The new feature, called Previous Versions, is enabled for users with the System Protection setting enabled. Previous Versions is similar to System Restore points but on a per file and per folder scale. It shadows copies of files whenever they're edited, allowing the user to revert to several previous versions via the right-click properties menu. In addition, users with this feature enabled can completely restore deleted files.
Now for the other side of the new feature - it gives Big Brother another way to check on you. Most likely people will want to have Previous Versions enabled for those "oh shit!" moments, but they will still be vulnerable to snooping.
As a result, it will essentially be possible to browse through archived filesystem states. For example, "Previous Versions" will allow you to open a historical backup of a folder to see all of its contents at that time. ArsTechnica