I have a differing theory. Kerry is talking about running again in 2008. That is not acceptable for Hillary! and her merry band of toadies in the MSM. So they will gradually turn up the pressure on Kerry until he folds and goes back to his accustomed role of being a backbencher in the Senate.
Russert just fired the first shot. Kerry now faces a conundrum. If he doesn't sign the SF-180, and stays in the race for the Dem nomination in 2008, the media will increase their pressure for him to sign and ask what he has to hide. If he does sign and stays in the race, then the media will then drip out his lies which could even jeapordize his re-election to the Senate.
His only real option is to not sign and remove himself from consideration in 2008 - then the MSM will forget he ever agreed to release the forms. It won't happen immediately - the MSM will give him time so it will seem that he wasn't pressured, because then the focus goes on them as well - but I would venture that, within six months, Kerry will announce that he is not running for prez in 2008.
A good theory and almost certainly correct.
MM
All it would take for him not to do it is for him not to receive the financial backing it would take.