What I would do if I where visiting, since they have no policy telling you before you register for your stay is, simply go about your business as if it were not prohibited, then if they make an issue, tell them you want your money back. two things are possible. Being a poor economy and knowing the competition for hotel guests in Las Vegas, they will cave in and let you carry in the hotel, for the purpose of coming and going to and from your room, or they will give you your money back. If they refund the cost of your stay, also demand transportation to a competing hotel. If you've already unpacked your bags I would demand that they send up someone from guest services to pack your things for you as well. Do what ever you can to be a pain in the ass without doing anything illegal.
DO NOT hand over your firearm to security! They will "loose it" for you. Stand your ground, make them understand that you have no qualms taking your business to another hotel. Excalibur is owned by MGM/Mirage their other properties include the MGM Grand, Mandalay Bay, Luxor, New York, New York, The Mirage and Bellagio. I work in a major Strip Hotel and as hard as this economy is on us, it's even harder on the hotel operators. It's almost impossible to be thrown out, and management is so scared that a customer will complain and get them fired that they will almost always cave in.
This is the most important thing! If they tell you firearms are not allowed this is not the same as ordering you to leave the premises. They must specifically tell you that you are being asked to leave for there to be any trespassing violation. If they do tell you to leave, immediately do so and call your credit card company and have them stop the charges.
I doubt it will come to that.