The reduction of civil liberty is very much like the frog experiment. For me the issue is ethical, not situational (which rules out objections like whether it's only temporary, or whether there are already safeguards in place). The government exists to serve the people, not vice versa. It shouldn't be given a questionable amount of power and then be trusted not to abuse it unless "necessary" to combat terrorism... that's such a vague and arbitrary line to draw.
Take the part of the Patriot Act that deals with online communication. (Sec. 217, Interception of Computer Trespasser Communications,
http://www.epic.org/privacy/terrorism/hr3162.html .) If I'm understanding it correctly, the government can legally intercept any online communication if a) the owner of the computer consents, and b) the user does not have a contractual agreement with the owner to be using the computer. Note the government does *not* need to get objective judicial oversight, which is not the case with similar laws in the past. Also note that if the information is never used in a prosecution, the user never even finds out they were being tapped, let alone has the chance to contest it in court.
Now this is something I'm not comfortable with even though my email and IM conversations wouldn't have evidence I've broken any laws. The Constitution does in fact include the right to privacy. (A good article naming some court cases where it's come up:
http://www.inq7.net/opi/2003/sep/20/...mentary1-1.htm .) If we're lucky, there'll be enough opposition to things like the Patriot Act that they'll never go very far. But the fact is that big changes come in degrees, and there've been countries that went in some pretty awful directions starting with small steps like this one. So it's a good idea to watch what you're really agreeing to.