Reminds me of the "It's a sin to vote for Clinton" line.
Of course it's as misplaced with Obama as it was for Clinton. Not that voting itself holds some moral quality, but that the issues surrounding and motivating that vote do indeed hold moral qualities. Government, as the Bible describes it, has a very specific purpose to it. As such we must find leaders who will fulfill that very specific job description. As such the question becomes: What's the purpose of government?
Scripturally the purpose of government is very very specific purpose. It isn't to save us gas money, or help the poor, or stop terrorism and make sure our troops come home safe, frankly. The purpose of government is to uphold justice and godliness. Look up 1 Timothy 2:2, and Romans 13. No mention of economics or the lives of soldiers. For Christians this purpose should be the fore-front of our decision making to vote on the figurehead of this nation. God says this is the purpose of government then we elect leaders to fulfill the role as such. Will he uphold justice? All other considerations, including arbitrary physical ones (one of the best Presidents was in a wheelchair), fall away. This is not to say issues are not important (though some issues the government shouldn't even be involved with, but that the issue of justice takes precedence, especially for us Christians.
So is it a sin to vote for Obama? I think it is. Not for such mundane and arbitrary reasons as race or a man's name. I don't think Obama is anymore evil then every other fallen human being, or that it's an inherent sin for voting for him. The sin is in our priorities. If they don't align with God's priorities, it is indeed sinful.
Will Obama uphold justice? I don't think so. One can feel comfortable basing this on the issue of abortion: The killing of those who have commited no crime that warrants it is indeed unjust. Obama has made his stand clear, and has shown a very unBiblical attitude to the issue of pregnancy. If one can't uphold such a basic concept, while claiming to follow the Bible the whole way, then I have no confidence in Obama's ability as a leader who upholds justice. Leaders should lead, not pander to the crowd.