I suspect you are very right. Very wise of them, too. Not sure if the source is Burkett, but I betcha the Bush folks know WHO.
IMHO, no way Burkett alone is the source of the memos. He was the designated third party stooge who was insurance in case things went south. These memos had their origination in the periphery of the Kerry campaign, and were passed through Barnes / Burkett to Robin Rather, then on to Dan. If Burkett was the source they would have fingered him sooner. Why risk the damage to CBS reputation if the source is a disgruntled guy who has had several nervous breakdowns. Why the delay in setting up Burkett, as is apparently happening now is puzzling though. Maybe there were some financial negotiations to be settled first.