I don't know the name, but the phenomenon you're describing is definitely real. People have a hard time acknowledging their inner racist tendencies sometimes, and they don't honestly consider themselves racist, so when the question is put to them directly, the answer is "Sure, I'd vote for a black person"... but then somehow their vote never quite manages to go that way. (And before anybody starts crying about this, no, I'm not suggesting that not voting for the black candidate automatically makes someone a racist).
The problem is that it's practically impossible to quantify this hypothesis. You can't poll people about it, because the entire premise is based on people (consciously or unconsciously) lying when they're polled. So all you can do is compare polls to actual results, and since polling is a very imperfect science to begin with, those comparisons aren't exactly reliable.
In this election, the place to watch will be Appalachia, where some white Democrats who voted for Clinton in the primaries may tell pollsters they'll vote for Obama based on party affiliation, but then pull the lever for McCain when they actually get to the booth.
But there also are reasons to believe that this might not be a huge factor in this election, since Obama's appeal clearly slices through racial lines. Granted, Iowa preceded Rev. Wright, but that went a long way towards dispelling the myth that Obama will have a hard time attracting white rural/working class voters. And then just yesterday this report from the
Washington Post suggests that even white working class voters favor Obama by a significant margin:
So when it's all said and done, my guess is that we'll probably see, overall, a white voter turnout for Obama that's just slightly less that what the polls were indicating going in. And that will fuel speculation and debate about whether some white voters lied in the polls and if so how many, but since there's no way to accurately measure that, we'll really never know.