If it is something like that, it had better be something bigger than just conventional adultery, which is far from being a deal breaker for voters these days -- recall the Republicans' effort to take down President Clinton over Monica Lewinsky, to which the public reacted mainly with exasperation that they were distracting the government with trivia. And it had better be credible. Romney has had the career of an economic parasite, but a philanderer or sexual predator? He just doesn't seem like the type. It wouldn't be easy for the average voter to believe. A hit story that lacks credibility would likely backfire and create sympathy for Romney.
Or the rumor could be just a rumor, with nothing backing it up at all. We'll see.
Update 1: A report from Britain suggests the Republicans are nervous about whatever it is.
Update 2: Birther attention-hound wingnut Trump is now saying he's got some dirt on Obama. Can't say I'm feeling very concerned.
Update 3: According to this article (sent by Parsley's Pics), Trump's big revelation is just some old Obama divorce papers.