Iran currently exports 2.5 million barrels a day of oil http://www.eia.doe.gov/emeu/cabs/iran.html#oil
As you can see in the below image supply and demand are currently very closely matched. If you remove 2.5 million barrels a day or a substantial fraction of that from the supply side the demand side won't be looking that pretty. Due to the global nature of the oil market even though we don't ourselves import oil from Iran, we will be affected. You like gasoline at $2.50-$3.00 now? Expect to see a minimum premium of $1.00-$2.00 on top of that if we invade.
