If a war occurs in Iraq, and it has UN approval, I doubt anyone will do much, other than the protests of those who feel either a) there is never any justification for war, or b) it's the evil West out to destroy Islam again.
Even if the US goes in unilaterally, I doubt any nation-states will actively take up arms in the conflict: I don't think anyone likes Saddam enough to stick their necks out. There may be individuals (particularly Muslims who see it as yet another attack by the evil United States on the Umma (I hope I spelled that right)) who decide to travel to Iraq and take up arms. I'm sure most Arab nations would make the most of it to direct the disaffection of their citizens at the US and away from their repressive regimes, and they'd likely impose some pretty hefty oil embargoes if nothing else. Whether the UN would impose any sanctions I don't know; I'm not familiar enough with how they work to know how one would get around the US's Security Council veto, if the US decided to excercize it in such a self-serving way, but if they don't at least try to they'll lose serious credibility.
Perhaps some particluarly anti-American folks out there might try to haul the US political and military leadership before the international war crimes tribunal. Wouldn't that be fun?
Here's hoping that it all remains speculation.
-Chris Landmark
"Was entstanden ist, das muss vergehen. Was vergangen, auferstehn." -Klopstock & Mahler
"Only liberals really think. Only liberals are intellectual. Only liberals understand the needs of their fellows." How much viciousness lay concealed in that word! Odrade thought. How much secret ego demanding to feel superior. - Heretics of Dune