It's indisputable that this was a serious speech about the incendiary topic of race in America.
Obama was performing his high-wire act, trying to appear black enough for the African-American community and post-racial enough for white voters. That's as tough a task as exists in American politics, and one speech alone will never accomplish it.
But if he is to win the nomination and the general election, he has to engage voters in this dialogue, and the sooner the better. He's started down that road, and he has to continue. Whether he likes it or not, and whether Americans generally like it or not, race is a big part of this election, and it has been at least since the election result in New Hampshire.
From Obama's perspective, it's much better to have this discussion now. In fact, the debate about race was inevitable at some point. I would argue that race isn't just another issue; it is THE issue of American history. He actually needs to have the racial debate continue until it exhausts the media and the electorate as a whole.
If he has to confront racial division in October in a major way, he will lose the election. By October, he needs to have the media and voters say, "We've already finished with this subject. What about Iraq? What about the economy?"