last I tried, which was a couple months ago, replacing the "www" with a "dl" in a public link made it work for hotlinking. so, starting with
https://www.dropbox.com/s/2s31p57eod49dva/1371724997329.png?...
and navigating to this
https://dl.dropbox.com/s/2s31p57eod49dva/1371724997329.png?d...
results in a redirect to this
https://dl.dropboxusercontent.com/s/2s31p57eod49dva/13717249...
which can be hotlinked. What this means in a specific sense, I haven't yet learned. I'm particularly interested to know the behavior of any of these links if the file is moved/renamed. I have not tested it, but my intuition is the dropbox.com link would survive a move and the dropboxusercontent.com link would not.