If you download a file with http or ftp it's obvious that you've "downloaded" it and where you downloaded it from, with a torrent it's less clear-cut, but what if the file just turns up in you Dropbox directory? I suspect this and similar situations are likely to keep lawyers busy in the future.
Case 1 is Dropship a software project which took advantage of Dropbox's file de-duplication functionality to "copy" files between different users' Dropbox accounts requiring only file meta-data (filename, hash) information. Dropbox have made api changes such that Dropship no longer works, but future projects that take advantage of the de-duplication loophole are almost inevitable.
Case 2 is Boxupus who also download torrents "directly" to your Dropbox account. I don't know the mechanics of how they do this, but taking advantage of Dropbox's de-duplication algorithms seems to be a possibility. They also have been stopped by Dropbox, but surely they or others will be back.
One thing is certain "downloading" and file sharing are not going away any time soon, and at some point it's going to be difficult to determine "where" the bits are, and "who" is actually downloading.