Agreed, and it helps to have 2 backups of all your data using different techniques/products. I run a local backup to cheap disk, network backup to remote location, and I suppose my remotely hosted svn is my third backup of most assets I really care about.

Most of us here are developers. How many of our products are perfect? Always assume something will fail in a new and interesting way in the future. I'm not letting Dropbox off the hook - their product shouldn't do this - but you'll be happier if you treat backups with the same level of redundancy and planning as the rest of your infrastructure.