I've heard rumors of arrests actually taking place, but upon further investigation I usually end up finding out that there are also other, more sinister things going on.
Here's what REALLY happens: They take it away from you and 86 you again. I'm basing this on personal experience; over the years it has happened to me or an acquaintance of mine, many times (more than twenty).
In all of these times, only ONCE did I ever see a casino try to push it any farther, by trying to have the person arrested. When the casino called the police, they were told: "It's too minor of an offense for us to send an officer. Just confiscate it and bar the woman".
Another time, as I was getting on a boat in Louisiana; I noticed a pile of about 20-25 fake ID's sitting on the security guards' podium at the entrance to the boat. I struck up a conversation with him about it. It turned out that they were all from underage people trying to get on the boat (he was quite proud of his ability to spot fake IDs

). In the conversation he did mention that they never arrest any of those kids; all they ever do is just confiscate the ID & send them on their way.