I'm sure Dropbox takes security seriously and works hard at it but this piece doesn't tell me a lot more than that except in much longer form. You can find/replace 'trust' (and 'security') with 'truck' and come away as informed and potentially slightly more amused.

The harder a company tries to sell their philosophy the less I'm inclined to believe them. Words are cheap.

a better approach would be implementing a zero-knowledge storage infrastructure, like tarsnap.