I have seen people get caught a couple of times. Once they were told to put it away, the other time the FA joked "I didn't see that". It's technically against federal law as Larry said, but I think enforcement is poor unless someone is being disruptive. With that said, I certainly wouldn't try it. I'm not sure how you would get more than 100 mL past the TSA anyway. I suppose you could buy it post-security.