This is an interesting question. I was at a talk at this years's GsoC summit where this was discussed and indeed, the language of the BSD license does not issue

This is an interesting question. I was at a talk at this years's GsoC summit where this was discussed and indeed, the language of the BSD license does not issue an `irrevocable` license as the language of a lot of other licenses does. Nobody has every tried revoking an MIT or BSD license before, but it would be an interesting legal argument to see if the courts would allow it.