IMO, his best pick is Richardson. He's suffucuiently liberal so that he wouldn't offend the left wing and he brings along ethnic strength and also opens possibilities in the SW. In addition, he's currently outside of the Beltway. Although there's not a strong anti-incumbent mood this year, I think a ticket of a sitting US Senator and another Senator or Congressman is just to much of inside DC to play well across the country.
Richardson would be my second guess for the Dem VP. While he is closely associated with the Clinton Administration, he doesn't exude the more negative qualities associated with that era.
But Richardson, I think, see the handwritting on the wall as it relates to Kerry's chances. It would hurt him with the voters in New Mexico, if for no other reason than they just hired him as Governor...and here he goes trying to get back to Washington.
Unlike Gephardt, who doesn't pretend he's going back home.
If Richardson is picked we'll hear endlessly about how wonderful he is because he is Hispanic and how bold and courageous Kerry is for making the pick (like Gore with the Lieberman selection because he is Jewish)...but as others have speculated on FR, Richardson may have a skeleton or two in his closet that would make him a liability.