We were there in November when it was being added to the various locations, and the woman at the "bar" in Three Broomsticks said it's permanent and year-round now. It's even been added to the menus.
We LOVED the hot butterbeer and wished we could have had it earlier in our trip! They served it in about a 12 oz cup, so about the size of a cup of coffee. The weather was very cold while we were there, so this was a nice option to get our butterbeer fix without holding a cold/frozen cup. The hot version was our favorite, followed by cold, then the frozen. The ice cream had a primarily vanilla taste and not much of a butterbeer taste, in my opinion.