Ah, but see, they're still mad at him for not wanting to kill ALL the brown people; he was a moderate on immigration until he started running for President.
I think I've already linked this, but
August has a great rundown of why Biden would be a good running-mate for Obama.
How fringey IS the fringe? Are these people who would stay home or vote Barr rather than McCain? Anecdotally, I know one guy who's going to write in Ron Paul rather than vote McCain, because he believes the latter is a RINO.
Regarding the talk about racism: it's not a binary value, guys; there are DEGREES of racism. While I'm sure there are a hell of a lot of people who won't vote for Obama no matter what, I'm sure there are some who would be more likely to vote for him if he had an appealing running-mate. But you ARE right that he shouldn't focus on whether his running-mate's race, he should focus on policy -- there are many issues that could hurt Obama in a general election, of which his race is only one. Biden or Richardson would be a great pick to make up for his lack of foreign policy experience, for example, which is VITAL in that McCain is running his entire campaign around his foreign policy experience.
There are other issues to consider, too -- southern states with large black populations ARE likely to see unprecedented black voter turnout. That may swing a few. And I think the uglier the racism gets, the more backlash we're going to see -- there are a lot of people, both Democrats and Republicans, who are embarrassed by racism within their respective parties and who would be more likely to vote Obama simply to distance themselves from bigots. Again, I don't see that swinging the election, but I think it's going to swing some votes.