From the FAQ (http://getdropbox.com/faq):
15. How public are Public links? How Private are my other files?
* Any file in your Public folder is accessible to anyone who can guess your public file link and the path to the file. No one can browse the directory, though.
* Any file in your Dropbox outside of the Public folder is by default only accessible via computers linked to your account or to you via the web interface.
* If you give out a shareable link to a portion of your Photo Gallery (from here), people who know the link will be able to browse any photos in the album you linked from and any sub-album.
I see photos and public when I first sign in - are these both public folders? How do I store things, lets say my resume I want to be able to access from anywhere, but want private?
Other than that, it appears to 'just work' amazingly. Having built a somewhat similar backend to a system like that, it really is quite a feat.
Edit: Since the startup I was at is now defunct - email me and I may have some a good potential market for your product (very slightly modified)