But, I believe you can synch a file to multiple computers through Lockbox. If your main working ftm file is in your Dropbox folder, it is being continuously synced to Dropbox - which, in turn, can sync to other computers. There is no networking involved.
I've not had the personal experience of doing this, just relaying how I understand Dropbox works.
Dropbox can only work with small file space (limit 2gigs for free) and then you pay an arm and a leg for webspace - way beyond my budget.
Of course, the simplest way of syncing with the Cloud is the Sync feature built into FTM2012 with ancestry.com. It advertises a lot cheaper than Dropbox.
If anyone has experience with the new Google Cloud Drive, people here would be interested in how it works and how well it works.