There's still a big problem with de-duplication: Dropbox can still figure out which users have the same file, thus leaking information. That, combined with the fact that they'll know the size of the file already gives them a lot of info.
For example, if the FBI seizes a computer and finds some illegal files, they can still request Dropbox to give a list of users that have the same file.
As has been mentioned elsewhere in the thread -- de-dupe isn't responsible.
If Dropbox is storing your files -- then the TLA can always request Dropbox to give a list of users that have the same file. (Unless you have some form of independant crypto/hashing)
For example, if the FBI seizes a computer and finds some illegal files, they can still request Dropbox to give a list of users that have the same file.