I think you might see Hillary jump - she has to remain a strong contender for 2008. If Kerry were to win, she MUST be the VP or risk fading into the shadows.
There's a couple reasons why I don't think Hillary Clinton will be the VP nominee:
1. Kerry will lose (I admit I'm biased against him, but this is my honest assessment), and Clinton doesn't want to be associated with a sinking ship.
2. Clinton will overshadow Kerry, and the race will be about her, for better or worse
3. She's a polarizing figure, and nothing will help Bush raise money or turn out Republican voters more than the prospect of Hillary Clinton being a heartbeat away from the presidency
It's also interesting to note that Clinton and Kerry apparently disagree on the biggest issue in the campaign - the war in Iraq. Clinton has been quietly supportive, for the most part, of the Iraqi war. Kerry is apparently opposed to it, although it depends on what day you ask him, or which quotes you want to emphasize.