By Howard Kurtz Washington Post Staff Writer Monday, August 11, 2008; Page C01 The whispered allegations about John Edwards were an open secret that was debated in every newsroom and reported by almost none. The story of Edwards's affair with a former campaign aide became so widely known -- what a Slate blogger called "undernews" -- that by last week there seemed little point in the mainstream media gatekeepers' keeping it isolated outside their moat. And yet, even as some national news organizations tried halfheartedly to confirm the tawdry tale, they ignored it in public -- wary of the National...