I thought Burkett basically admitted to being the source when he said that "we have reassembled your records", and that he in turn took them to Cleland after getting the run around while trying to contact the Kerry campaign.
So if that is the chain of custody - Burkett, Cleland, Kerry campaign - then obviously Rather got them from the Kerry Kamp. Burkett has already taken away the possiblity of denial for CBS. They have nowhere to run or hide now.
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.