Pew Research demonstrates the double edge sword of celebrity endorsements. Accoring to Pew, Oprah's endorsement of Barack Obama would have "no effect on vote" for 69% of surveyed folks, but 60% thought it would help Obama's candidacy. In other words, it won't change my mind, but I'm sure it will change somebody else's. Interestingly, Oprah has more pull than Bill Gates, the local newspaper and less than the clergy, and Alan Greenspan.
In the celebrity endorsement camp, an endorsement by Bill O'Reilly is likely to negatively affect twice as many people as it would positively effect. Likewise, an endorsement by Jon Stewart would be a more likely to influence 1 voter for every 1.5 voters it didn't. Kanye West and Donald Trump? 1:4.
Also interesting is the influence of endorsements of the press: the same percentage of people would be influenced by the local paper positively as negatively, according to Pew.
The most important finding of this research is that between 61 to 79% of people would not be incluenced by anybody's endorsement--that speaks volume for either a) the educated voter b) the uneducated voter. I hope it is the former and not the latter.