Haaretz correspondent Shmuel Rosner, writing in the The New Republic, argues that Israel might attack Iran not to destroy its nuclear program -- which it probably can't do -- but to force the international community to act:

The main goal of a hit would not be to destroy the program completely, but rather to awaken the international community from its slumber and force it to finally engineer a solution to the crisis. As one former Israeli official put it, any attack on Iran's reactors--as long as it is not perceived as a military failure--can serve as a means of "stirring the pot" of international geopolitics. Israel, in other words, wouldn't be resorting to military action because it is convinced that diplomacy by the international community cannot stop Iran; it would be resorting to military action because only diplomacy by the international community can stop Iran.

I don't believe this is Israel's first option. More likely, Israel's threats are intended to ratchet up the pressure on Iran to compromise. But as Rosner notes, "The more Israel pledges to stop Iran, the more it becomes necessary to deliver." If you keep crying "wolf!" and nobody listens, the best way to get people's attention is to shoot the wolf.