When I store things in my Dropbox account, it's usually just to transfer between my home and work computers. It's things like artwork I use for backgrounds, grocery lists, a short story I wrote, wishlists... basically, things I don't care if anyone sees.
I do store some sensitive stuff there, too... but it's encrypted. think of it like email; anyone between the sender and the receiver can read the email, if they wanted. That's what PGP is for, right? Encrypt your email, and no one else will read it. All you need to do is encrypt your data. Oh, sure, someone might be able to crack it, but if it's something that is that important... don't put it in Dropbox.
I understand the concern - Dropbox should make sure that user files are already encrypted - but for goodness sake, people. If you're concerned about security, shouldn't you already be encrypting any files that aren't chained to your wrist?