Dr. Katz is in part 2 http://www.youtube.com/watch?v=NCLeh2MJZf0
Also, there was an episode where Arthur turned 12 and was afraid of his parents not treating him like a kid anymore, and they had a bunch of references to "grown-up" movies like Misery and Silence of the Lambs.
My job at the SPED classroom involves a lot of PBS kid shows.
I only *wish* Arthur was on during our rest times. What I have to watch every weekday is Barney and Caillou. Oh God, I hate Barney so much, and Caillou is an idiot.
Arthur seems like it should belong on Nickelodeon or Cartoon Network, and not in the same block on the same channel as the Teletubbies and Sesame Street.
It's also one of those shows where you get the inkling that the writers have a lot of fun with it. It pokes fun of itself and PBS so many times.