Obviously I am no expert, but I do not believe Israel sits idly by. They will let the negotiations, etc, play out. Once those are exhausted it most likely will be go time. I don't see that they will have an alternative unless something is worked out in the international negotiations. The longer they wait, the tougher it will be. The bottom line is that I doubt they wait to see if Ahmadinejad keeps his word or not...
One would never know it from the mainstream media, but Iran has not launched a war of aggression fo centuries. To the contrary they have been victims of aggression many times. Israel, by contrast, has repeatedly attacked its neighbors
Also Israel has fought all its wars against Arab states. They have at times been friendly with Iran.
An unprovoked attack on Iran would assure that nation's total hostility for generations to come. And Iran has 13 times the population of Israel.