I’ve been seeing a lot of Dropbox folders pop up here at the lab. This makes me nervous for you users. If you establish an account with Dropbox from a public place, Dropbox will ask you to download its application. Doing this sets up a permanent folder at your computer. If you do this, the application grants access to your files to anyone who sits down at said computer. Even after you leave it. Not good.
You can still sign up for DB at a public computer, just don’t download the application. Save that for your personal computer. You cannot upload the same amount via the web interface as you can with the application, but you still have far more space than you would if you were trying to e-mail something to yourself.