Due to Dropbox's implementation of de-duplication of identical files, any user can (in theory) determine whether (but not who) some other user is storing the same file. If you upload a file that any other user has aleady uploaded, your file transfer will be nearly instantaneous.

See: "How Dropbox sacrifices user privacy for cost savings"

http://paranoia.dubfire.net/2011/04/how-dropbox-sacrifices-u...