Evidently in the movie (Forgetting Sarah Marshall) there's a scene with puppets designed by Jim Henson's workshop which is pretty hilarious. That may have had some role in securing them the gig. I guess we'll find out when the movie hits theaters April 18th and we can see their puppet work for ourselves.
I think this could be pretty cool. I mean, the last few muppet movies kinda sucked and this might be a way to infuse some fresh ideas from some truly funny guys into the franchise.