Kurtz at the Washington Post summarizes the press's dilemma on the Edwards story: on the one hand, it's more than reasonable for the media to exercise caution when a tabloid breaks a political sex scandal (anyone remember the Kerry intern story that turned out to be bullshit?), but on the other, there comes a point where there's enough evidence to do some real digging and determine whether the story's for real.
In this case, the National Enquirer got it (at least mostly) right, and the MSM dropped the ball.
(As for the Edwards story itself? It leaves me thinking much less of him as a person, but that has nothing to do with his politics, which I still wholeheartedly agree with. But it was irresponsible for him to run for President with baggage like this that he should have known would eventually come out.)