I flew from NYC to San Juan, Puerto Rico that is within US border, and I was asked for a passport at JFK. But it is only when you check in. You need not to go through customs or anything similar.
Thanks, Natali. I think that bears some investigation, maybe on the TSA web site. Puerto Rico is a U.S. territory, but--rightly or wrongly--I'm not sure that a flight there, "over seas," from New York might not be considered different from a flight from, say, New York to Los Angeles. But I could be wrong.
There were lots of news stories on TV about air travel over the just passed Thanksgiving holiday, with lots of talk about how to pack and prepare for Security, but I don't recall anything being said about needing a passport for a domestic flight. Nothing was said about needing one for the upcoming Christmas travel season, either, but maybe this regulation will come into effect in the new year.