There was a minor splash several months ago when Cliff Schecter's book, The Real McCain, reported that McCain blew up in front of three reporters in 1992 when his wife Cindy was teasing him, and called her the C-word. McCain's campaign denies it happened, but McCain himself refused to answer whether he'd done it when asked at one of his town hall meetings (to listen to him, go to On The Media, start the June 13th show, and advance to minute 11:30).
So where does that leave us? In some cases a refusal to answer is an admission, but in this case it's a harder call. The US News link above states ambiguously that it tried to track down the sources: "An effort to arrange to speak with Schecter's sources was unsuccessful, though the author described in some detail the positions held by the sources at the time of the alleged incidents and their whereabouts today." My guess is that US News thought they had enough info to figure out who the reporters were, but the reporters wouldn't talk.
So I think the ball's in Schecter's court, with the exception that I agree with Mark Kleiman that there's enough legitimacy for the mainstream media to force McCain himself to answer the question. Schecter, meanwhile, needs to go to his sources and say that the campaign has denied it's happened therefore they should do the right thing. Schecter also needs to ask his sources what prominent newsperson they'd trust to preserve anonymity, and get that person to interview them. I've also scrolled through Schecter's prolific blog postings - he mentions the issue once, but hasn't been able to move the ball any further.
One sidenote is how Wikipedia should handle this. It's been discussed and shot down repeatedly at wiki, apparently. I brought it up in the talk section for one article. At this point I wouldn't include it in wiki, but I'm less sensitive to feelings here at the blog.
Finally- if the prominent "newsperson" was Oprah Winfrey and she got it confirmed, that would pretty much be the end of McCain's campaign.