According to The Prowler's "Anatomy of a Forgery", the DNC got these documents "more than 6 weeks ago", and back then, Kerry wasn't doing so bad in the polls. The Swifties were making waves, but I don't think Kerry or the DNC had reached the panic stage yet. So it seems pretty clear, before that timeframe, someone had hatched this dirty trick, prepared the documents, and was scheming to carry it out.
I don't think Carville or Begala or any high profile spinning official would have the smarts to create these documents. Remember the secretary who worked for Mark Mckinnon? A mischief making nobody, who mailed Bush briefing papers to Gore Hqs. Someone like that is probably responsible for the fraudulent documents. The hoot is that CBS, all caution to the wind, went with the story.