Good point, the only way CBS can make a clean break is to do a story on how the documents came through the Democratic party. I think the likelyhood of that is less than zero.
I think this is what will happen:
1. ol' DandyDan will float some completely unsubstantiated allegation that they were deceived by a right-wing partisan trickster.
2. This will give the Dims a chance to continue to claim that GW didn't meet his obligations.
3. CBS will refuse to reveal their source even though that source somehow 'deceived' them.