I'd like to visit the USA in four weeks and recently read about the "Visa Waiver Program Improvement and Terrorist Travel Prevention Act" which disallows people who visited Iran since 2011 to travel to the USA without a visa.
Does this already apply? If not, when will it apply?
You may have heard of talk about H.R. 158, the "Visa Waiver Program Improvement Act of 2015" (which may also be referred to as the "Visa Waiver Program Improvement and Terrorist Travel Prevention Act of 2015"), which has not been passed by Congress, but you are really looking at the wrong bill. This bill has not passed because it is no longer being considered. Congress decided to pass the provisions through a different bill because politics is confusing like that.
Changes to the Visa Waiver Program have indeed been recently passed, as part of H.R. 2029, the "Consolidated Appropriations Act, 2016" (the "Omnibus" bill), and was signed into law by the President on December 18, 2015. The changes to VWP are in Division O, Section 203 of the bill. The gist of it is that people who have traveled to Iraq, Syria, Iran, or Sudan since Mar 1, 2011, or who have the nationality of Iraq, Syria, Iran, or Sudan (regardless of whether they have ever been to that country), are disqualified from using the Visa Waiver Program.
People who don't use the Visa Waiver Program are not affected by these changes. (For example, some countries are not part of the VWP but their nationals can visit the US without a visa: e.g. Canada, Federated States of Micronesia, Palau, Marshall Islands, and, obviously, the US.)
Update: The US announced that it will begin implementing the changes on January 21, 2016. It says that people who currently hold ESTAs who have indicated having one of those 4 nationalities will have their ESTAs revoked. It says that the Secretary can waive the restrictions for certain reasons but it's not clear where in the ESTA process such a waiver would be applied for.
Not yet, and it might never apply - but to keep tabs on it:
H.R.158 - Visa Waiver Program Improvement Act of 2015 - the bill's current state.
It's passed the house, and has been received by the Senate, but hasn't been voted on there yet.
As many have observed, let alone tourists to the US, it may create two classes of US citizens, for those who have visited Iran.
As a result, it might be ruled unconstitutional, but we'll see. Stranger things have happened.
As a NZ citizen who visited Iran last year, I'm also watching it with interest and alarm.
These changes are not yet in effect. They will not take effect without an announcement. See, for example, http://america.aljazeera.com/articles/2015/12/30/changes-to-visa-waiver-program-could-impact-ngos1.html:
The changes have yet to be put into practice, according to the Department of Homeland Security (DHS), which is responsible for implementing the VWP provisions.
“At this time, no determination has been made as to how the waiver provision would be implemented,” DHS representative S.Y. Lee said in a statement. “We will announce any changes affecting travelers to the United States from visa waiver program countries as soon as that information is available.”
The DHS has not provided details on when that information will be available or indicated if exceptions will be made for aid workers.
UPDATE, 22 January 2016:
Unfortunately, it appears that the announcement was made yesterday, with immediate effect. Earlier assurances that "an announcement would be made," implying that people would have adequate notice to adjust their travel plans, appear to have been disingenuous or incompetent.
