Starting recently, the United States government has made a major shake up on the travellers eligible for the Visa Waiver Program (VWP). Previously people were allowed to travel to the US for 90 days on an ESTA (or what I call a visa which isn't a visa) if they were citizens of one of 38 countries.
Well now your eligibility is revoked and must apply for a US visa if either of these two conditions apply:
Nationals of VWP countries who have traveled to or been present in Iran, Iraq, Sudan, or Syria on or after March 1, 2011 (with limited exceptions for travel for diplomatic or military purposes in the service of a VWP country).
Nationals of VWP countries who are also nationals of Iran, Iraq, Sudan, or Syria.
So, excruciating bad luck if hold nationality with, or have travelled to, any of the four countries singled out as the source of terrorism (funny how Afghanistan, Israel or Palestine isn't there right?). If you previously had an ESTA you may find that that has been withdrawn – you should check the official ESTA website long before you travel.
If you are a Dual US national with any country (including the aforementioned four) then nothing changes for you as by law you must identify yourself to be a US citizen when you arrive anyway.
Whatever you do, don't fall foul of the rules like this British-Iranian BBC journalist . Granted she sounds like she tried her best to get all the information she could.