Aside from Hutch's spot-on observations, I find the possibility of Obama ordering a shoot-down of Israeli fighters to be completely unrealistic. The political fallout would be absolutely disastrous for him and the Democratic party. That would cause a mass defection to the Republicans - not just from the Democrats' massive support from the Jewish community, but also from the moderate/conservative Democrats. They would, in effect, be handing over the next election or three to the other side of the aisle.
That's not to say Obama would allow Israel to attack Iran without consequences; as others have noted, to do so would effectively allow Israel to declare war on Iran on the US' behalf, and that's simply a horrible, horrible idea at the moment. Even in the snowballs-chance-in-hell possibility that the US would remain neutral, the very last thing Obama needs is a war fought literally over Iraq.
Also, rhetoric aside, I find the possibility of Israel launching a unilateral attack against Iran to be fairly unrealistic as well, unless Netanyahu has completely lost his marbles.
Even aside from Iraq, an Israeli strike force would have to go through one or more of the following countries: Turkey, Syria, Jordan, and/or Saudi Arabia - none of whom would be willing to allow them to do so, and with the exception of Syria, they all have modern US-built fighter jets (to say nothing of surface-to-air capabilities). This would, quite probably, start a regional war--and even if the US was 100% behind Israel, frankly we can't afford it. The Iraqi "government" is shaky at best, and Afghanistan is making Iraq look positively utopian.
In short...Beat your chest all you want about Israel having the "right" to attack Iran, but you better damn well hope it doesn't happen. Nothing, and I mean nothing, good would come of it.