Ever since the Egyptians threatened to shut the Suez Canal and the Straits of Tiran in the 1950's & 1960's, the United Nations has deemed the closure of an international waterway by any party to be an "Act of War" as basically it threatened a country's trade and economic well being.

Counter to what Charlie Noble says the Straits of Hormuz are 21 nautical miles wide at their narrowest point (water depth between 75 and 100 metres). The both shipping separation lanes through the Straits are entirely in Omani territorial waters. Two pipelines by-pass the Straits, one to the Red Sea and another to the coast of Oman, I dare say that in the light of these threats of closure other pipelines would be built making passage through the Straits to load redundant for anything other than Iranian oil.

The Iranians have tried using mines in the Gulf before, in April 1988 they damaged a USN Frigate - US response when it was established beyond doubt that the mine was Iranian was the largest surface action fought by the USN since the end of the second world war - It resulted in the loss of one Iranian Frigate and six of their fast patrol boats, another frigate was damaged and two offshore platforms used by the Iranian Navy for command and control purposes were destroyed.

IF (and it is a BIG IF) the Iranians were stupid enough to attempt closure of the Straits of Hormuz I think the closure if it succeeded at all would be extremely short lived and that they would be in for the greatest shock of their lives. It is not the movement of US assets into the Arabian Gulf that Iran wants to look to as an indicator in fact quite the reverse.