Keep an eye on Khuzestan. That's the South-Western Iranian province, bordering Iraq and the Persian Gulf, that currently has a modest but growing independence movement. It was also the site of the recent bombings that Iran is blaming on UK influence. Within the US intelligence community, at least, it is widely believed that the best option for dealing with Iran is through fomenting internal unrest of some sort. The classic formula for this (see "Coup D'Etat: A Practical Handbook" by Luttwak) is to leverage existing internal devisions--and that is exactly what is happening here. The US is actively supporting this Khuzestan independence movement, and the various "Free Iran Movements" that are being supported by right-wing think tanks in D.C. have many ties to this region. Not surprisingly, Khuzestan is the major oil producing region in Iran, but the revenues don't provide much benefit to the local and ethnically distinct Arab population. You may also recall the Iranian Embassy siege in London in 1980--also the work of the a group from Khuzestan agitating for autonomy from Tehran. So there are definitely some genuine tensions here for US exploitation. This is, of course, highly speculative, but I think it is still worth considering: the US may not need to invade all of Iran to influence their choices--they may just need to help the people of Khuzestan break away, and "help" a pro-US government set up shop. The mere threat of this--if the ground work is put in place to make it very credible--may be enough of a negotiating piece to force Ahmedinijad to give in to US pressure on a wide variety of issues.
Do a search on google with any of these terms and you'll see how this little-publicized issue is quickly catching on with many Washington think tanks and other "tree-top" policy projects... take a look, for example, at the Ahwaz Studies Center in Lorton, Virginia... smacks of Ahmed Chalabi in 2002??