The film is no overcaffeinated Narnia-like religious allegory, though. Horton and the mayor aren't hearing things. When the burden of proof falls to each of them, they scramble to produce hard evidence. And it's a nice touch, one not in the original story, that the first person to believe the mayor is a scientist. "Horton" may owe a debt to the notion of childlike belief, but any movie that fills Whoville with ladders that look like DNA strands and shows sound waves pulsing through the atmosphere is a movie that celebrates reason. It's probably not a coincidence that it's from the creators of the "Ice Age" films, which gave pretty wide latitude to Darwinism.
When scientists and skeptical thinkers are portrayed as pooh-pooh killjoy wet-blanket snotty/snobby wrongheaded villains in just about every Hollywood picture, that's really nice to hear.