I feel like I've had to show my passport to the gate agents (along with my boarding pass) boarding international flights.
However, I haven't flown internationally in a few years, so I don't remember too well.
A friend of mine who has flown internationally recently, said that they never asked for his passport at the gate.
My reasoning is that if I turn out not to have a passport and immigration in the arriving country don't let me in, the airline will be responsible to take me back to the US.
I'm specifically wondering about travelling out of the US into Canada, but curious to know for other countries (or is it an airline policy thing?).