Pssst! Quiet now and don't tell anyone... but they're already there.
Most likely on international flights but also on seleceted domestic flights.
Having been a commercial pilot for many years I can tell you they are always identified to the flight crew, but noone else. They began right after 9/11 here in the U.S., in earnest, athough they had been used prior to that. Almost every country has our equivalent of Air Marshals. But... unless their very specialized services are required, you'll never know one is setting next to you in 14 C..
Have a nice flight!