Dropbox effectively acts as an "existence oracle". You can't ask it to cough up a file you don't have, but you can ask it if a given file exists anywhere in the system.
This would be an effective way for law enforcement or copyright civil enforcement to check for content that is clearly illegal or a certainly copyright violation to possess. They would need to query for a set of hashes of the given illegal content. If any matches returned positive data, they would be able to issue a subpoena for all users who stored the given content in their dropbox folder and pursue them further.
How can something be "clearly" a violation? If I have an album, but copy someone else's rip instead of making my own - is that "clearly" a violation? Alternatively if I used the same application, I'd probably obtain the exact same file - is that clearly a violation too?
(grooveshark kind of operates on the assumption that it's ok)