Supposedly, Word of God declares the Clone High Janitor and the Scrubs Janitor to be the same character.
It all makes sense... Glenn acts so nice on Clone High, but the trauma of being fired as well as the death of his son, combined with the trauma of suddenly being in live action, could naturally have shattered the Janitor's mind and left him in the bitter/insane condition that he is in throughout Scrubs. Penny in the door=litter?
This theory is validated on the season 8 finale of Scrubs, in which Janitor reveals his name to J.D. as Glenn, the name used by the Clone High character.
Bill Lawrence's wife Christa Miller, who plays Jordan Sullivan, voices Cleopatra.
The background of the credits is a coloured closeup of the center of the golden ratio.
Donald Faison (Turk) plays Joan of Arc's blind father Toots, George Washington Carver, and literally every other black male character.
Zach Braff (JD) makes a cameo in one episode as Paul Revere.
John C. McGinley (Dr. Cox) plays a creepy trucker in one episode.
Sarah Chalke (Elliot) appears in a couple episodes as Marie Antoinette.
The X-Stream Blu promoters? JD, Elliot and Turk.
And from Spin City, Michael J. Fox had a very minor (but fully credited) cameo as Gandhi's Remaining Kidney in the first episode.
MADtv fans should recognise Michael McDonald note (not to be confused with the singer. Michael McDonald was MADtv's longest-running white male cast member, known for his recurring character Stuart, an overgrown kid who lives with his alcoholic mom, played by Mo Collins, and celebrity impressions of Hugh Laurie [as Dr. Gregory House], Chris Hansen, Dr. Drew, Sarah Jessica Parker, Criss Angel, Larry David, Dr. Phil, Maury Povich, Steve Jobs, Jeff Probst, and others. He is to MADtv what Darrell Hammond, Fred Armisen, Phil Hartman, and Bill Hader were to Saturday Night Live, in terms of doing more celebrity impressions than any cast member who has ever been on the show) as Gandhi.
No Export for You: Averted to an extent in the United States as the Canadian DVD release can be legally imported into that country and play on all standard (NTSC) DVD players, as Canada is also in Region 1.
Word of God: This interview with the creators reveals a few things. The overall plot they had for the series was for season one to be junior year, season two the first half of senior year, season three the second half, and at the end they would go through a wormhole and repeat senior year in season four, and if there was a season five, they'd make it college.
It also states that season two could've started out with the three main characters walking like in the pilot (except to senior year), having forgotten what had happened over the course of the summer, and putting the pieces of the puzzle together to find out how they got out of the cliffhanger.
Also, due to the controversy of Gandhi's character, the creators had to pitch two alternatives of the show to MTV, one where Gandhi mysteriously disappears and turns out to have somehow sacrificed himself in the meat locker to save his friends, which would just altogether drop his character from the show, and the other where they keep the character while writing in that there was a mix up in the lab and Gandhi is actually the clone of Gary Coleman.