She won't announce till the last possible minute.
The Clintons are supporting Edwards. If he is nominated, he will either Arkancide or facts will "come out" which make his election impossible, in either case around 10/15/04.
Hillary cannot stand the heat of a real campaign. She has to come in late, take those RAT votes that are fixed (the "base"), and count on sympathy for the rest.
If she cannot engineer the nomination of a loser, she will have to take it herself, in which case the minute she drops in the polls I fully expect X42 to Arkancide, swinging a large sympathy vote to the grieving widow.