Blogger Doug Thompson of Capital Hill Blue was recently shown an FBI document declaring him to be an enemy of the state:

The printout, shown to me recently by a friend who works for Justice, identifies me by a long, multi-digit number, lists my date of birth, place of birth, social security number and contains more than 100 pages documenting what the Bureau and the Bush Administration consider to be my threats to the security of the United States of America.

...

Although the file finds no criminal activity by me or members of my immediate family, it remains open because I am a "person of interest" who has "written and promoted opinions that are contrary to the government of the United States of America."

Much of the FBI's information was obtained through "national security letters" (example at right), which solicit information, but forbid those be asked for information to disclose that they've been asked.

Source: Capital Hill Blue (via Cryptome).