The Bradley effect is a supposed effect that Black candidates poll numbers are often higher than the percentage they win in the actual election. One explanation that is offered is that survey espondents do not want to appear racist or respond with an unpopular answer and thus skew the results.

Articles about this appear at salon.com and fivethirtyeight.com.
I agree with the sentiment of Nate Silver, but I think his analysis is lacking. A better analysis of the problem is found here in a paper written by Postdoctoral Fellow Daniel Hopkins.

Hopkins concludes in his paper:
“The Wilder effect occupies an unusual position in our thinking about American elections, as it is often invoked (e.g. Elder, 2007; Lanning, 2005) but rarely scrutinized. By analyzing Senate and Gubernatorial elections between 1989 and 2006, this paper has provided the first largesample test of the Wilder effect. In the early 1990s, there was a pronounced gap between polling and performance for black candidates of about 2.3 percentage points. But in the mid-1990s, that upward bias in telephone surveys disappeared. At a time when scholars are increasingly across states.

At a time when scholars are increaseingly concerned about the validity of phone surveys, these results provide some reassurance. We have also seen that the polling-performance gap is closely related to a candidate’s level of preelection support, meaning that we should not naively attribute the entire observed gap in a given election to racial bias. Douglas Wilder, David Dinkins, and Tom Bradley were all frontrunners, and so all could have expected a small decline in their election day performance. Even
female front-runners should expect declines into election day, although they are not subject to any Wilder-style bias.” – Hopkins (2008)

Sort of unrelated note:
Hopkins in his article says, “Even Tennessee’s 2006 Democratic nominee for Senate, Harold Ford Jr., experienced no Wilder effect after a negative television advertisement targeting him cued anxieties about inter-racial sex.9” The footnote number 9 is, “9. Specifically, a white actress in the late-October advertisement exclaims ‘I met Harold at the Playboy party’ and then closes the advertisement by winking and saying, ‘Harold, call me.'” Amazing.