Using dropbox is a bad idea overall in my opinion. They lied to their customers about what they can and cannot view in your account. They lied to you. They intentionally told you that they could not view your files when in fact they can. If that's not enough, they had a critical security vulnerability (log in with no password) for four hours, proving that their systems fail open. Finally, if all this wasn't bad enough yet, they do not encrypt your files when they store them[1].
[1] Technically they do encrypt the files, but the keys are right next to them on the same infrastructure. Doesn't do any more good than not encrypting them.
From a security conscious perspective it is a bad idea, but too many people undervalue noncritical data and don't secure it like they should (That's part of why infosec is such big buisness). A prevelant lack of security sense is basically the only reason dropbox is still in business.
But from a HN perspective they've received YC cash so your post is currently voted <= 0.
For the bits that need to be protected there is Tarsnap (http://www.tarsnap.com/). The client is open source, so you can check for yourself that things are encrypted before going on the wire.
[1] Technically they do encrypt the files, but the keys are right next to them on the same infrastructure. Doesn't do any more good than not encrypting them.