Certificates are like bitcoins, you cannot reverse a transfer, that's what makes the whole system powerful. In the future, you'll be able to make conditional transfers on Bitproof (based on time for example: let's say this file belongs to XXXX for Y days). Is it a feature you would like to see implemented soon? (If yes, to do what kind of business?)
The only use case I can think of that is of course a bit-DRM, which frankly I'd rather no one tell the RIAA or MPAA about.

It's very interesting though, I'm sure the right application will come along for it.