How it could have gone down:
'Someone' creates the memos, uses Clealand to get them to Burkett and also arranges for the information to get to CBS that Burkett has a 'smoking gun'.
Mapes goes to Burkett, gets the memos, ID's Clealand as his source. CBS calls Clealand, confirms, and runs with the story.
Story falls apart, CBS asks Burkett to change his story about who gave them to him (or just agree to say he changed his story), Burkett agrees.
CBS ditches the memos on the chain of custody excuse rather than the forgery excuse, and avoids revealing either chain of custody beyond Burkett, thus covering for the 'someone' who started it all.
Yea, but Burkett and Cleland will never confess to anything (at least until after the election). Imho Burkett/Kinko's is the probable source of the memo's (bfd), B had motive & opportunity, shopped them around to willing suitors, and got a bite at cbs (mapes) who ran with it. Fallout is that cbs gets drubbed by other tv "news" organizations for weeks, hurting the Kerry "campaign". Dan wanted to help, but in the final analysis (ok, well, not yet) hurt Kerry's campaign. (Although that shrew Teresa hasn't helped much, either - "scumbags!").
Grig, I believe you nailed it.