I simply can not believe that a racist would support a candidate of a race they don't like. Hell, if they did they would not be a racist, since then they'd be making that decision based on something other than race.
Without weighing in on whether or not this is actually at work in this particular situation, I'd like to say that Twixlen's underlying point is correct: It is entirely possible, and in fact quite common, for people who are extremely racist in general to have particular people of the the hated race that they are very fond of. For example, in the slaveholding South, many of the white plantation owners were famously adoring of their "Mammies," the slave women who did the actual work of raising them. That didn't, however, change their overall relationship with their slaves.
It's long been noted that racist people seem to have a particular affection for black people who seem servile, buffoonish, deferential or otherwise non-threatening. Whether that dynamic itself would be enough to lead to an actual vote by a racist for a black presidential candidate in a general election is something I have grave doubts about, but it doesn't seem like this election will actually test that in one way or the other.
I'm also not sure I'd actually put Cain in that category, even though he didn't seem to be working overly hard to be taken seriously. He certainly wasn't any more buffoonish than most of his rivals.