Issue 1: Differences of opinion on what Israel refers to as the Immunity Zone. It is the point in time when Iran’s accumulated know-how, raw materials, experience and equipment (as well as the distribution of materials among its underground facilities) — will be such that an attack could not derail the nuclear project of Iran. Israel believes that the window for action is nine months (so by mid-September). The US believes that the window has 15 months so until late May 2013. These calculations are based on what each country believes that their military can deliver in destroying facilities etc. hundreds of feet down in the earth. (Some data shows Iran has gone down as low as 250 feet) So Israel’s technology will be unable to achieve the desired result if they delay in acting past mid-September, all things remain constant. It would then mean that as a nation they are then going to have to make preparations for the reality of a nuclear Iran because they will be unable to stop it, again all things remaining constant. BTW - it was reported within the week that the bunker buster bombs the US currently have are not powerful enough to go down past the two hundred feet range and that the US has placed orders for more powerful versions of the bomb.
Issue 2: Is the matter that the US wants notice before Israel actually strikes. Israel to date has said they will not make that promise but sources believe that Israel actually would provide the US about 1 or 2 hours notice which is enough time for them to make some preparations but not enough time for them to derail the military strike.