I know this thread is kinda old, but I was reading and Anit Dev said something that I really needed to reply to.
If your primary focus is civil liberties the answer is never. If your primary focus is security the answer is when ever necessary.
"Those who would give up Essential Liberty to purchase a little Temporary Safety, deserve neither Liberty nor Safety"
I think this is an extremely pertinent point, especially with regards to the new war on terror in the US. In other words, revoking civil liberties is simply un-American to me.