I doubt that Shiite militias in Iraq can or will do anything much just because Washington bombs Iran. Most of them work for Washington anyhow. Governments of the EU and East Asia _won't_ do any damage to the US economy _purposefully_ -- that's like cutting off your nose to spite your face.
The only effective brake on Washington is the concern that some members of the US power elite may have about what sanctions, missile strikes, or a coup against the Ahmadinejad administration might do to oil supplies and prices: "If the UN Security Council voted for sanctions against Iran, 'it would mean an enormous disruption as oil prices might rise to over 100 U.S. dollars (per barrel),' said Joseph Stiglitz, who won the Nobel Prize in 2001" (at <http://english.people.com.cn/200604/06/eng20060406_256217.html>).
That sort of concern may not be enough to dissuade the Bush administration. After all, it was willing to destablize Venezuela (the fourth-largest supplier of oil to the United States), despite a possibility that its destablization might have a more immediate negative impact on the US economy than anything that it might do to Iran.
I suspect, however, that the most likely scenario may be that Washington will simply try to kill or oust Ahmadinejad (e.g., having Iranian exiles organize a car bomb attack on him while he's visiting Germany). Khamenei, Rafsanjani, et al. will probably make a deal with Washington. You see, nukes aren't really the issue for Washington -- that's just a card; the Ahmadinejad administration's economic program and foreign policy is. Whilst Ahmadinejad does have military experience like Chavez, a plus for his prospect for survival, I don't know what kind of following he has in Iran's military. Enough to prevent or survive a coup? -- Yoshie <http://montages.blogspot.com/> <http://mrzine.org> <http://monthlyreview.org/>