We've been hearing this one a lot, lately.
(The "suicide pact" metaphor is from a 1949 dissent by Justice Jackson, but he was talking about the Bill of Rights, in the future tense, in a conditional clause. Justice Goldberg, in 1963, put the sentence substantially in its modern form in his majority opinion in Kennedy v. Mendoza-Martinez.)
The thing is, it's a damnably ambiguous statement. You can read it forwards, as saying that the Constitution is a wise document, one that promotes both liberty and security, and therefore adherence to the Constitution will never create a risk of suicide. Or you can read it backwards, as saying that to cling blindly to the Constitution's literal statements in times of grave emergency would be tantamount to signing a suicide pact, and therefore it must be read as containing an implicit exception clause for use in grave emergencies.
Of course, the implications of these two readings are exactly opposite. Which makes it perfect rhetoric, because it's so manipulable to serve whatever present interest needs serving. No wonder that judges and pundits love it so.
The Ashcroftian meaning is in the ascendency these days; people haul out the quip when proposing new curtailments of civil liberties. Perhaps it's time to stop conceding the meaning and insist that true security requires the constant protection of the liberties enshrined in our Constitutional order. Tossing aside two centuries of accumulated respect for basic freedoms seems like a pretty dumb thing to do, one might say. Perhaps even suicidally dumb.