The Horton quote "I meant what I said, and I said what I meant: An elephant's faithful one hundred percent," is actually from "Horton Hatches the Egg," another Dr. Seuss story featuring the enduringly faithful elephant.
The Mayor receives a goldfish in a fishbowl from a Who who looks remarkably like the Cat In The Hat (he is only seen twice, briefly - once from behind, and later from a distance - and his hat has blue stripes instead of red). Later, just before the first attack by Vlad, the Mayor is resuscitating the fish. When the fish lands back in his bowl, he leans on the side of the bowl just like the goldfish in The Cat In The Hat.
This quote which was spoken by the character Katie in the movie: "In my world, everyone's a pony and they all eat rainbows and poop butterflies," is often incorrectly attributed to Dr. Seuss. In fact the line was written for this film. The quote and the character Katie were not written by Dr. Seuss and were created after his death.
Amy Poehlers character is named Sally O'Malley, which is also the name of a recurring character on "Saturday Night Live (1975)" played by Molly Shannon. Both Poehler and Shannon got their careers started on Saturday Night Live.
In the scene where the Mayor of Whoville consults Dr. Leroux about what would happen if Whoville was a tiny spec in space, you can see that Dr. Leroux is holding up a piece of paper. On that paper, the first paragraph is just random typing. However, the following paragraph, it reads: "Clerk Maxwellies which othe machines, ranging fromag-neticles variety. Aside tranging. Electrodynamics at rea of theorology of the for can ever..."