I caught it as well. If you missed it though, don't feel too bad as it wasn't spectacular; it was fun but nothing to write home about. Still, since it's a pledge drive special, those who missed it may still want to check their local PBS listings to see if it will (re)air later this week or up til Christmas.
The Muppet segment was just about five minutes. Kermit. Piggy. Fozzie, Gonzo, Animal, Pepe, and some penguins join Andrea on stage and the Muppets ask what they should sing together. Andrea suggests Jingle Bells to which the Muppets enthusiastically agree. The orchestra begins a very legato intro (which causes the Muppets to look around at each other wondering if its the right song). Andrea sings a very slow dramatic version of it til Kermit pipes up that the rest of them feel it should be substantially more uptempo. The Muppets pick up the tempo but Andrea still sings his way slow. Throughout the performance, Piggy is smitten with Bocelli and insists that he's perfect and can do no wrong when the other Muppets want Jingle Bells to be poppier. As Andrea still sings slowly, Piggy sighs that his version is very romantic and the others start agreeing it's not so bad this way.
What stood out for the Muppet Geek part of me was that it seemed that these were all live vocals (Andrea and the Muppets) meaning that three of Eric's characters were onstage at once (and Eric obviously was only puppeteering one) meaning he would have been alternating between voicing the others while two other puppeteers would be lip-syncing to his vocals on the spot.