No inside info here, just an educated guess.
Since the Top Ten segment typically is topical current event humor, the Top Ten they were going to do with the Muppets probably got bumped to another date to accomodate a list the staff wanted to do right away to tie in with something from the days headlines. Assuming the original info on the appearance is from a reliable source, im guessing that the Muppet segments were pretaped so the show could end up using it whenever they wanted to so it might not be a bad idea to set those vcrs the rest of the week and maybe next (or until we get an update from an insider).