According to this article from Slate, President Bush has officially authorized U.S. forces to kill or capture Iranians fighting with Iraqi insurgent militias in order to stop Iran's influence in Iraq.
If U.S. forces on the ground in Iraq are attacked by Iranians, I expect, nay, encourage them to return fire and defend themselves. There's no reason they shouldn't be able to protect their own troops. What concerns me is that this could potentially be seen as authorization to pursue and attack groups that definitely are from Iran- potentially throwing gas on the Persian-American fire that is already building.
Iranian insurgents fighting amongst Iraqis is not only possible, it's probable. I would have preferred that soldiers just did what it took to defend themselves without having an officially sanctioned policy that target Iranian citizens.
While this policy is not the same as declaring war against Iran, it still does mean the killing of another nations' citizens... and the Bush administration insisted this wasn't an escalation! It's only a matter of time until enough Iranians are killed or captured that Iran officials get really upset about this and does something drastic, or mobilizes their military to defend themselves in Iraq.