You are almost certainly aware that Hillary Clinton has not released her tax returns (she says they'll be released "about" April 15th). You probably don't know that John McCain hasn't released his, either; this is one of many examples of McCain's good--one might say cozy--relationship with the press.
Talkingpointsmemo notes a number of aspects of the story that ought to make it interesting to the MSM.
The Democrats are going to have to dust McCain up and puncture his image as Mr. Honesty. Fortunately, there's plenty to work with here, and McCain's notoriously short temper is likely to create further opportunities. We're not talking about slander here, just getting the whole story out.