Look, how can anyone turn down an honor like that, honestly?Upside? Former congressman. Ambassador to the United Nations. Secretary of Energy. Governor of New Mexico. He's a Mexican-American and would almost guarantee Obama a super-majority of the hispanic vote in November, and might even put Texas in play for the first time since 1976.
Downside? Did you see him run for his party's nomination last winter? He can come across as a bumbling fool from time to time, giving the Republicans plenty of fodder.
Putting Richardson on the ticket is definitely worth considering, however I think he'd be more useful, in what promise to be some pretty rough post-Bush years, back at the United Nations.