To some extent, I think questioning Obama about Ayers is legitimate. If the leading candidate for President has been "paling around with terrorists" than that is surely the nation's business right? It certainly deserves more of an inquiry than whether Trig is really Palin's son, the matter which still--amazingly-- seems to be so important to Andrew Sullivan.
So it's not the inquiry itself that makes the current Ayers attacks so deeply shameful; rather it's the fact that, at this point, there appears to be nothing more than the most tenuous connection between the two men. CQ politics has an excellent run down of the claim that Obama and Ayers served on the board of a "radical" educational reform organization. It doesn't cover the other commonly asserted concern that Obama launched his campaign at Ayer's house, but it's worth reading, if only because it will let you assuage any concerns that friends or family may have.