The problem in Iraq was that the US underestimated the Sunni/Shia split. The Sunni were in power under Hussain, but the Shia are the majority, so now they have power, hence the conflict. If you look at the headlines now mostly the killings are between Iraqis. Very rarely do you see American casualties.
In Iran the situation would be very different. I have a feeling that the majority of Iranians would welcome a US invasion. The Iranian people are suffering from the harsh autocratic rule of the Ayatollahs. The people themselves want a new, more moderate government.
