I've never come across a candidate who has been the subject of more unsubstantiated whispering among the press corps. Some paper with real resources needs to send someone to New Mexico to track down the start of the whispering campaign against Richardson, and then either knock it down decisively, to give the man the fair shake he deserves, or publish the story of whatever its basis might be. It is completely absurd, even Kafkaesque, to have a substantial portion of the national press corps convinced that the man ought not to be taken seriously based on unsubstantiated gossip.
I'm sure it'll happen. I'd add that there are two factors making this a bigger issue for Richardson than it might have been a few elections ago. One is the way the Internet has eroded taboos of political discourse (the sort of rumor once contained mostly to Georgetown dinners and the Washington Post newsroom now trickles out on blogs). The other is that fact that in the very early nineties everyone in political media was hearing about Bill Clinton's penchant for womanizing but mostly shurugged it off--only to see it become a defining element of his campaign and nearly destroy his presidency.