If we first strike Iran, they will immediately stop all oil exports to the US and Europe and close the Straight of Hormuz effectively cutting off virtually all middle east Oil. It will be virtually impossible to stop them from doing this. Hugo Chavez in Venezuela will probably halt all oil exports to the US. The price of oil will go into triple digits to who knows where overnight along with the distinct possibility of a world wide recession / depression. This in addition to all hell breaking loose in the middle east.
The alternative is very probably a nuclear armed Iran within a few years.
One scenario that looks all to plausible:
http://www.realclearpolitics.com/articles/2006/09/deterrance_is_a_mere_wish_in_i.html
"Ain't nobody getting out this alive Doc....." - Al Swearingen