Israel has already warned the US they won't wait until that point, and our game plan until now has been to do what's necessary to keep Israel's hands clean. Besides, it's necessary that they know exactly whose boot is on their neck.
I just do not think that there is the polictical will to strike Iran, the chances of igniting a world war by doing so is just to great. At best the resistance in the middle east would grow to more then the US could manage. A strike on Iran might very well result in a US defeat in the middle east. Far to risky until Iraq is a stable and pro west state.