Dave Greenbaum points out, quite well, that syncing is not the same as backing up. 

http://theappleblog.com/2010/03/26/syncing-does-not-a-backup-make/

It is a really great post, which notes the distinction between making something available in multiple places and actually backing up.  MobileMe, DropBox, and Sugarsync, for example, are not online backup services, they are sync services, and while they offer some potential to use file versioning to retrieve files, there is no guarantee!  Of course, Mozy doesn’t guarantee anything either.  But some folks do, like The Backup Plan