An attack on Iran would not serve Americans Interests
It seems like Bush is trying to soften up the country for an attack on Iran -- taking an aggressive tone in the issue over nuclear development (contrast that to his almost total lack of initiative regards North Korea, especially where Korea has actually tested nuclear weapons, and Iran insists they are only trying to develop nuclear power, whether you believe them or not). Then there is the US government report of weapons from Iran being used against US troops in Iraq.
Clearly, there is a long history of tension between the US and Iran. Not to mention Iran's involvement in Lebanon, etc. It would not be surprising if they are arming Shi'as in Iraq, not so much because they are anti-American, but because they would much prefer a Shi'ite regime next door (for obvious reasons, since Iran is itsef a Shi'ite theology.)
Whether Iran is arming Shi'ite militia in Iraq or not (and frankly after the weapons of mass destruction lies/debacle, I take anything the US government claims when trying to justify an attack on another country with a huge grain of salt), an attack on Iran would really not be in the US's best interests. Even Bush's belligerance is not helpful.
Ahmedinejad's party just suffered a defeat at the polls, but with the country under attack, those reversals have been ebbing, and support for his regime has risen again. An attack, even the mildest of bombings, will galvanize that support. (You can imagine how support for Bush would jump if, say, Venezuela sent planes across the ocean to bomb one of our nuclear facilities out west). An all out attack would only make that support stronger, but it would make anti-American sentiment grow, not only in Iran, but across the world, exponentially.
And you can imagine the mess if we invaded Iran and toppled the government. You think Iraq is a morass, Iran would be no better. Indeed, it would likely be worse. Clearly, that would not serve our interests in the region. And increased hostility towards the US will ease the tasks of terrorist groups, both in recruiting and in their ability to evade authorities as more people sympathize with their sentiment, if not their methods.
Down at the bottom of the list has to go the impact such a move would have on the US economy. We are already way over spent. We have already had to cut back our own social programs. Our educational system, medicaire, social security -- all those programs that are struggling -- could have benefitted from the monies spent in Iraq, and will only suffer if we invest in attacking Iran. Imagine the impact we could have if instead of spending billions of dollars killing people and creating mayhem, we had invested billions of dollars in making the world a better place -- helping to build infrastructure in countries that have none. So too, we should take the money we would spend attacking Iran and use it for better ends.