Recent news is floating around about Dropbox preventing users from sharing copyright protected files. The keyword is sharing as apparently this policy has been in place on Dropbox for some time. Only after a user attempts to share a file does Dropbox scan it and determine if it is a known copyrighted file. If so, the share is removed but the file is not changed.
Dropbox does this to prevent their servers from being used to share illegal files between two people. Apparently sharing illegal files with yourself is still ok! For more details check out this good article.