the problem is not the reality of any threat posed by iran, but that iran as target fits neatly into the neocon fantasy of themselves as avengers of previous wrongs done to the american hegemon--iran is obviously as big a deal in that world as iraq, but the logic is a bit different: it fits neatly into the question of their favorite phantasm, "islamic fundamentalism" (which you see repeated ad nauseum in the various space within which conervative rationalizations for the actions of a basically pathological administration get deployed).
selling the war to a credulous tv-viewing public would not be as problematic as selling a war unrelated to these already existing terms would be.
thing is that invading iran would be a total disaster.
a much bigger disaster than iraq has so far been.
on the other hand, bush, like his 1920s-1930s ideological predecessors, seems to require constant war to legitimate his policies, and his administration more generally, before the public. this is the other logical space that makes anyone who thinks about it worry a bit that this neocon fantasy may well end up generating alot more deaths for very little reason.
on the other hand:
http://news.bbc.co.uk/2/hi/americas/4185205.stm
it appears that the only population on earth that finds this administration to be coherent, its policies admirable, are american conservatives. the only population that finds this policy of absurd war in iraq to be reassuring, and who would potentially welcome an even more idiotic invasion of iran, are american conservatives. it is not in the least surprising to find this same population totally incapable of relativizing its positions--like their boy bush, they appeal to an arbitrary higher authority, one that enables them to imagine 51% is an overwhelming mandate, an affirmation of policy blah blah blah.