Airline policies change, so check before taking your scanner. Used to be Delta allowed them, but last time I flew (a few months ago) their regs did not permit radio receivers in the passenger cabin. And that's been the case for several years now. To my knowledge, all other commercial airlines don't permit them, either.
However, United does many times offer "cockpit audio" on aircraft that are equipped with in-flight audio and headphone systems (for movies, music). In fact, they used to list in the inflight magazine that channel 9 on the audio system was communications audio from the flight deck. However, whether it is on or not is the pilot's discretion. When it is on, I listen to it the entire flight. Waay better than the movie or music if you're into aircraft comms.