Were there Hijackers? - (or Conspiracy Theory 101)

I started to write this as a comment in reply to a comment appended to George Washington's excellent post, but I decided to I see that this subject confuses many, so I will weigh in with a few observations. Before I do, some important reminders for us "conspiracy theorists":

http://omniadeo.blogspot.com/2008/05/conspiracy-theory-101.html