You have forgotten one real possibility - that this is a Clinton leak.
Bill Clinton admitted that he has known about this for some time. John Kerry claims (and I believe him this time) that he heard about it when we all did. If Clinton leaked it then the reason is obvious - deep six John Kerry rather than the Sandy Berger story and make plans for a glorious Socialist future in 2008 with Her Majesty, Queen Hillary. The media is more loyal to the Clintons than to Kerry and they will go along. This may have been Hillary's first shot across the Kerry (and Edwards) bow.
Interesting - and quite possibly true - The leaker I have thought from minute one is definitely from the DEM's - I've always thought it was Berger himself (trying to fend off any possible indictment that could be coming) -
Could it be a shot across the bow from the Clinton's? - Very possibly - more power to the evil bastards if it is!