According to a poll by Rasmussen, 22% of all Americans and 35% of all Democrats believe that George Bush knew about the 9/11 attacks in advance.
Perhaps that's because it's not at all unusual to hear bizarre theories about 9/11. If you spend a lot of time on the internet, you'll hear everything from Bush knew about 9/11 and let it happen, to Bush was behind 9/11, the Mossad was behind 9/11, the Pentagon was hit by a truck bomb, the Pentagon was hit by a missile, the World Trade Center was taken down by explosives, WTC 7 was taken down by explosives, mini-nukes were used on the WTC, United Airlines Flight 93 was shot down by a missile, United Airlines Flight 93 was never shot down at all, etc., etc., etc.. In other words, any and every crackpot theory that you could come up with in your wildest nightmare has probably been floated by some lunatic or attention seeker on the Internet.
So why have these conspiracy theories managed to spread? In large part because most serious commentators usually think it's beneath them to actually take the time to respond to the conspiracy theorists. The problem with that is that the nuts end up dominating the conversation by default because the sane, knowledgeable people tend to opt out of the conversation.
The other big problem is that conspiracy theorists use a style of argumentation that tends to baffle a lot of people. What the "truthers" and other assorted nuts do is ignore the big picture and focus on small things.
You see, there are always stories that are gotten wrong by the media in the aftermath of a big story (Think about how badly the Hurricane Katrina coverage was blown), small inconsistencies, and loose ends that aren't tied up. What the conspiracy theorists do is bring these minor issues up and demand that people explain them or else admit that there's a conspiracy. Most people don't know how to handle that because, quite naturally, they don't know what temperature steel melts at, who said what to whom two days after 9/11, or anything about some obscure study that the conspiracy theorists cite.
But, here's the thing: the conspiracy theorists have it backwards. It's the conspiracy theorists who need to build a case that explains what happened better than the official version, not people who believe the coherent, accepted version of events who need to explain away minutiae that the kooks have come up with.