Still, that the number is greater than zero is significant. Why would someone who backed Clinton in the primaries now prefer McCain to Obama? Possible reasons:
They want to vote for a woman; Sarah Palin on the Pub ticket made all the difference.
They don’t want to vote for an African-American, and would have gone over to McCain whomever he had chosen as running mate.
They still believe in Clinton’s politics and proposals, and believe McCain represents them better than Obama does. I mention this only for the sake of completeness; I think it can be ruled out. The number of voters who think this way would have to be vanishingly small.
Probably in many cases the answer is a combination of 1. and 2., but that’s just my WAG.