Calling it "The Daily Deli," Arby's John Stewart tribute sandwich keeps it simple with a double portion of corned beef served with deli mustard on marble rye bread. While it won't appear on the menu, you should be able to order it by name at any Arby's restaurant.
As a bit of background, Jon Stewart has consistently mocked Arby's for the past few years on The Daily Show. The chain has apparently taken it all in stride though, as you can see from the farewell video they put together for him:
Here's the video introducing the new sandwich: