sigh Dropbox should not be used as a backup system. A system that synchronizes live should never get that role, except if they can guarantee that old data is never overwritten, new is always appended. This is not the case with dropbox - I've experienced multiple scary occurrences of old versions going to the nirvana with certain user actions. In certain cases the old data just appears to be gone, in other cases the web interface shows them but a restore results in an error message. Especially data move appears to be buggy. Dropbox appears to be simple, but the backend processes are really not and there is too much going on for it to be a reliable backup system, especially if you also share some of your data to team mates.
edit: emphasis formatting, niceifying.