When we checked in at POR on the 2nd, the CM told us they were only good for our resort (which we already knew). When we were waiting for the DME to go back to the airport, I heard a CM telling the customer that they could use them at any of the resorts. We heard two different things just at our resort. Granted, no signage, but....
I will say that this trip, we saw lots of different mugs. And, while I didn't take it with me from resort to resort, I don't see why you couldn't use them at the other resorts if you were leaving MK for lunch or something. Lastly, I'm hoping that it's something that can be reused from trip to trip. We now have three sets of mugs from our last trip alone. I like them but I don't need that many. DH will become as attached to them as he did the cracked Sprint cup that I threw away that had a chip in it that he just after he started working. There was no reasoning with him on that one!