One plausible explanation:
1. Burkett and Dem operatives give photocopies of the forged memos to DNC/Kerry camp.
2. DNC/Kerry go to CBS to float the story, give photocopies of their photocopies for the story.
3. CBS sits on story until timing seems opportune (last week).
4. CBS gets source from DNC/Kerry camp, goes to Burkett who faxes them a better copy off the originals from Kinkos last week.
5. Document reviewer sees the Kinkos number and leads us to the Burkett connection.
That still leaves the Kerry connection out there and we should explore it more. But it also makes sense that CBS would try to obtain a first generation copy of the forgery direct from is source (which Burkett appears to be) rather than running one they got second or third hand as it filtered through the DNC and Kerry camp. Perhaps there was a phone call of some sort: "We've seen a copy of your document from the campaign but we need it direct from you for the story. Can you fax it up?"