Some of the First Amendment rights were clearly eroded after 9/11 such as the government's right to investigate what is on computers at the public or school libraries. As an indication that there are things that are forbidden to say in the U.S., a few years ago Swiss tennis pro Martina Hingis complained that two American players who are sisters were given an inordinately high percentage of commercial endorsements and other pros made no commercials. Later, she was severely berated as making racist comments. So, she came on American TV and apologized, saying, "I am sorry; I am not from America; I did not know your laws."
When there are "unwritten laws" that prohibit people from criticizing others without losing their jobs, positions, etc. then there no longer is freedom of speech. After all, the very term "politically correct" points to the fact that people have lost certain freedoms. Recently in the news, a football coach lost his job because he wrote a song that criticized negatively the president of the US. A few years ago, a prominent (and liberal) news magazine (Newsweek or Time) published an article a couple of years ago on how many Conservative Christian college students in various universities across the country who expressed their viewpoints in the classroom were graded poorly by their liberal professors.
With the erosion of First Amendment rights, there is, indeed, the possiblity of people's eventual arrests for saying or doing something that disagrees with those in power. Or, before this happens, we will have more of what already exists: the media destroys a person's good name with innuendoes, etc. After all, America has already had its Red Scare, has it not?