That's such a great museum! They also have a "build a muppet" workshop, and a bunch of cool interactive exhibits! Big recommend for anyone visiting Atlanta.
They didn't have that workshop when I was there, but there was a lot of great Henson stuff! Apparently they really rotate things a lot, because I was talking to a friend of mine who went there a couple of years ago, and what they had on display was totally different.
But as far as I know, our two boys are on permanent display.