First, and most importantly, the Iranian nuclear program is not an easy, localized target. It is dispersed in multiple locations throughout the country, and probably hardened to resist attack. The Iranians are well-aware of the possibility of attack, not just from Israel, but from the U.S. I don't overestimate Iranian defensive abilities, but they will do whatever they can to protect those sites. Even poor efforts could be significant given the range from Israel, and the necessity for a sustained air campaign. The military difficulties involved in destroying the Iranian program are daunting, even for the U.S., which has far greater resources than Israel.
Second, Israel has to weigh the costs of a preemptive strike against the benefits. And finally, it has to consider the costs and benefits of not attacking, but of pursuing a policy of defense & deterrence instead. For the first equation, it is the possible cost of not attacking that is most dramatic and often emphasized. That risk is that Iran will not only acquire nuclear weapons, but use them against Israel, or provide them to terrorist proxies. This is rightly seen as an existential threat to Israel. But how serious is that risk? It is extremely unlikely that an Iranian or terrorist first strike would be able to eliminate Israel's nuclear deterrent, especially given the probable location of some nuclear weapons on Israeli submarines. If any significant Israeli nuclear force remains intact after an attack, Iran would suffer a devastating nuclear response, possibly great enough to make a first strike on Israel equivalent to Iranian national suicide. How likely is it that Iran is willing to risk national suicide in order to destroy Israel? Should Israel operate on the basis of what is probably an extremely small risk factor?
If an Iranian nuclear first strike is deemed so unlikely, and I think it should be, then it is also unlikely that the benefits of an Israeli attack outweigh the costs. The probable costs are huge. An Israeli preemptive strike clearly does not have the backing of the Obama administration. How will Israel's greatest ally and protector react to an Israeli policy that not only goes against the direct wishes of the U.S. government, but also might plunge the region into war, destabilize Iraq, divert U.S. attention from Afghanistan, drive up the price of oil, and incur all sorts of other disastrous potential consequences? And that's not even considering the possible direct cost of Iranian retaliation against Israel itself -- all for an attack that most experts think will be able to do no more than delay the Iranian nuclear program.
In contrast, a policy of defense and deterrence seems to offer a far better cost-benefit calculus. By maintaining the capacity to retaliate with overwhelming force, combined with the degree of protection offered by its developing multi-layered missile defenses, Israel should be able to deter any Iranian nuclear attack. For this reason I think it is unlikely that Israel will strike Iran. In order to attack, fear of the worst-case scenario, a suicidal Iranian regime that launches a first strike, has to take precedence over a cold cost-benefit analysis. That's always possible. It may be that Israel will decide that no matter the consequences, no matter if all they are able to achieve is a delay in Iran's nuclear aspirations, that they simply can't risk a nuclear Iran. I know there are people who see things that way, both here and in Israel. I guess we'll have to wait and see.