If Kerry is looking for a guy with reformer credibility such as McCain or - as Kos posted - Dean, one obvious and less controversial option would be Wisconsin's Senator Russ Feingold.
He's not a Republican like McCain, so the base won't revolt. He doesn't have the "scream" baggage (even if the baggage was undeserved) and high negatives that plague Dean. He has the reformer credibility that excites some people about Dean, McCain, and Nader. Plus, he's from the battleground state of Wisconsin.
He's been around long enough to pass the credibility test. The media likes him, AFAIK, and he's inextricably linked to John McCain, which doesn't hurt with moderates.
The more I think about, the more I like it. I'm probably missing some really obvious negative, so fire away!