Your concept seems ridden with so many problems and possibilities or abuse that I'll let this as an exercise to the reader... (example: how do you "tag" existing files? existing CDs? whose interest would this be? would people bother? etc.)
To tag existing CDs, we've already seen that problem solved with iTunes Match.
To get people to do it, it would be a part of uploading media files. If SOPA were to pass, I imagine hosting sites would require only "signed" media. It wouldn't be perfect, but any distributed file would be signed at creation. Self signing would be simple enough if it were built into media programs. The key to all this is that nobody controls it, and the standard is open source. Once again looking at BitCoin as a role model.
Also, I don't think it's fair to tag a concept "ridden with problems and possibilities for abuse". It's a concept, not an implementation. And it was a mighty vague one at that.
Your concept seems ridden with so many problems and possibilities or abuse that I'll let this as an exercise to the reader... (example: how do you "tag" existing files? existing CDs? whose interest would this be? would people bother? etc.)