If you are a foreigner in the USA, intending to fly domestic, ALWAYS carry your passport.
I faced great delays yesterday, while trying to catch flights between New York and Atlanta.
Before security, the TSA checks people’s boarding passes and IDs. They accept US Driving Licenses as valid IDs. But they were less than impressed by my UK Driving License.
At NYC La Guardia, I presented my UK License, and the lady immediately gave that back to me and demanded a passport. I was at a loss. I didn’t imagine I’d need to carry a passport. She then made me stand aside for 10 mins while she located her supervisor. The supervisor looked at my UK License, my credit card and my business-school alumnus ID. Then she made me go through ‘extra screening’ — which meant they swabbed my hands for residue (or something) and did some chemical test on it. But then they let me through OK.
When I tried to get on my return flight at Atlanta airport, the process was repeated — except they were adamant that NY security shouldn’t have allowed me to fly without a passport. I was made to sign some disclaimer giving them the right to dig up my personal data, they noted down my addresses, looked at my license, my business school ID, my office security pass and finally let me go with a stern warning.
So, in short, when in the USA, always carry your passport.