My guess is they will not be attacks on his color per se, but will be an attempt to portray him as anti-white. This tactic will come from several angles: making it seem that he is connected to radical anti-white groups, playing up the anti-white rhetoric at his former church, and that sort of thing. In addition, I have heard a rumor that there is a 30-minute video of Michelle Obama saying some things about black-white relations that, released at the right moment, could be very damaging. The truth is that black people have had a really raw deal in the US and there is a lot of very justified bitterness in the black community. With the right spin, this bitterness could be portrayed as a reason to not vote for a black candidate.
There are some people in the US who are not willing to vote for a non-white candidate. Once they get to know Obama and hear him talk, many of those people will come around and drop their prejudice; some will not. But I don't think that's the real threat.
The real threat is that "conventional wisdom" will become that President Obama will only represent the interests of black people or that he has anti-white sentiments. The fact that this is hogwash is irrelevant. Many people believed, at least for a few months, that John Kerry was not a war hero but was in fact a coward. Many people believed that Gore was a delusional liar. For some reason the most successful negative strategies seem to be the ones that are the opposite of the truth. Obama is a leader on racial cooperation, but may be made to seem divisive on racial issues.
This sort of ghastly reversal of the truth works for two reasons. One: timing. It has to be done late enough in the campaign that the candidate doesn't have time to effectively counter the slander. Two: it has to dishearten the candidate's supporters. Even if they don't believe the lies, their enthusiasm is muted; their support weakens; they may not show up on election day.
The Democrats are really in the driving seat this year. We are set to sweep congress and the White House. We should be trying to foresee this sort of swiftboating tactic in advance so instead of being shocked, we can rise up and yell "Bullshit!"