Hundreds of years of scientific insight have little effect on the worldview of millions of US citizens. A large fraction of US-Americans believe the earth was created after the Sumerians invented glue or that Noah's Ark transported millions of gigantic dinosaurs in pairs. "So what?" you may wonder, "to each his own!" While this may be a very tolerant attitude, the actions of people who disregard scientific knowledge may get you more upset. In the US, for example, about half the population is expecting the end of the world within their lifetime. Not surprisingly, these people care more about, say, gay marriage than about a sustainable lifestyle. Given the voting power and energy-hunger of 150 million Americans, such religious fundamentalism becomes hard to tolerate.
Along these lines argues a recent article in the magazine NEWSWEEK. Go ahead and read it, you'll start questioning your tolerance, I promise.

UPDATE: The journal NewScientist has a corroborating article in which the author details how the strong and growing evangelical movement is working hard to send their children back into the superstitions of the dark ages. A truly chilling read.