The FBI maintains a list of people who are basically not allowed to fly. There have been complaints that this list is politically motivated. Thus far, most of the people that I've seen complaining about being on this list actually ARE commiting serious crimes for political purposes. Whether or not its reasonable to be afraid of flying with someone who is burning university research farms because they oppose genetic engineering is certainly questionable, but the people in question are far from innocent. Either way, I agree with the ACLU that such a list should be a matter of public scrutiny, and there should be clear public guidelines on what it takes to get on the list. The potential for abuse is extremely high. I don't execpt that anyone on this site has been barred from flight, but its a reasonable idea to keep a note of this link in the back of your mind in case you run into someone who is.