I don't think Israel is overly worried about Iran's reaction.
In 2007, they were in indirect peace negociations with Syria, Syria has a more powerful army than Iran and is right next door. Yet the IAF still attacked the Syrian nuclear reactor.
There is a line of reasoning, which I agree with, that Iran is in a transitional state. You have a shrinking group of aging religious leaders in control, while the mass of the younger generation is more secular and open to the West. You saw that clash break out in the open after the last election.
There is a window of 5-10 years until the current leaders die off or lose control. The greatest risk is during that window and anything which is done to delay Iran's nuclear program past that window is seen as being in Israel's security interest.
The trick, of course, is how do you carry out an air strike without alienating the younger generation of Iranians? Obviously, anything that would cause a large loss of civilians is unacceptable, but a surgical strike of 1-2 facilities which are choke points for the entire program could be an acceptable risk.
As to what the Iranian reaction could be, it is hard to gauge. Iran and Israel are already in a technical state of war. Iran already funds and supplies paramilitary groups which carry out attacks against Israel. Iran's air force does not have the capability to strike Israel and they would need the consent of neighboring countries to let the Iranian army through. A real threat could come from Iranian scud missiles with poison gas/conventional explosives, but Israel already lived through that in 1991 and again, it could be an acceptable trade-off.
Again, if you look at the Syrian example, Israel imposed a total news blackout after the raid and was very careful not to crow or rub the Syrian's nose in it after the sept. 07 strike. Syria lodged a formal protest, then let the whole thing quietly die without taking any further action.
Having said all that, an airstrike is really a last resort. The Mossad has already been waging its own secret, dirty war using dirty tricks, sabotage, bribes, intimidation, even possibly assasinations to disrupt the program. It is very secret, but you see occasional hints in the news media. It is not pretty and leaves a bad taste in everyone's mouth, but sometimes, there is no other alternative.
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