I think Greplin could provide this service fairly easily (since they are already indexing all of your data from other services for you and perhaps have access to your DropBox account.) Just using Greplin today as-is they could be a good cloud-based "backup" of a lot of your cloud-based data, so long as they do not automatically wipe out older data to match what is on the individual services.
Another service that could be really helpful for this problem would be one that would just automatically burn your DropBox account (which ideally had copies of your other cloud-based data) onto DVDs and send them to some location at some interval for automatic cloud-based offline backup. Not sure if you can automate the mailing part, though, and I would worry a little about ensuring that people get the right data.
Another service that could be really helpful for this problem would be one that would just automatically burn your DropBox account (which ideally had copies of your other cloud-based data) onto DVDs and send them to some location at some interval for automatic cloud-based offline backup. Not sure if you can automate the mailing part, though, and I would worry a little about ensuring that people get the right data.