Jon Stewart himself guested on The Daily Show to talk about Congress' shameful failure to extend or to make permanent a compensation fund for first responders to the 11 September 2001 attacks.
Stewart graciously and correctly never tried to upstage Noah. The trouble was, he couldn't help doing so. Noah's bemused outsider never seemed more genuinely outside, less relevant to the show that now bears his name. The appearance did Stewart and the worthy cause he espouses good (I hope), but it had the deeply unfortunate effect of rendering Noah a nonentity. I can't see how Stewart can appear on the show again until or unless Noah has acquired the presence to stand alongside his predecessor without seeming lost.
Steve Carell's appearance on The Late Show with Stephen Colbert was a much happier experience all around. The two old friends swapped stories and praised each other to the skies, but never let too much time elapse between jokes. If it wasn't quite the comedic gem that Carell's triumphant return to The Daily Show was in 2005, it still made for thoroughly entertaining TV. (I'll even forgive Carell his totally believable but completely fake reluctance to sing with Colbert.)