There would be two triggers for Israeli action.
a. enrichment above a "policy level" (probably discussed with Washington and Nato members, and the 5 nations involved in the negotiations with them like China, and Russia).
b. delivery of the S-300 air defence system (that Russia is now delaying, on grounds that the US is engaging in negotiations over Ballistic Missile Defence in Europe with them).
Iran says to unveil air defence equal to Russia system | World | Reuters
If the S-300 is delivered then you'll see Israel strike before its operational, as with it currently only the US has planes able to evade it
in the F-117, B2 and the F-22 (as the F-35 isn't operational yet due to delays).
Certainly its volatile, expect to see more mining of the straits of hormuz, kidnapping and arrests of foreigners (due to trumpted up "Border violations").
The way the Yanks could deal with it, and they may do directly - would be a mass cruise missile strike along with B2 and F117 and F22 strikes on the nuclear and air defence facilities.
The US has already started installing more Patriot and Arrow 2 systems in allied Arab states.
They've also started sending more Ticonderoga-class Aegis guided missile cruisers in which also have anti missile capability to the gulf.
