Well worded question.
In my opinion, the real question here is "is there ever a case when one's rights should be suspended for the greater good of society". The follow-on question is "who gets to make this decision, and on what grounds".
This is an extremely dangerous "slippery slope". Convicted criminals are sitting on death row, only to later be proved innocent by new evidence. It's exceedingly rare that one can be 100% positive about anything involving criminal behavior - unless you already know much of the plans and actions, in which case you'd already have the information needed to resolve the situation without the information held in the suspect's head. Kind of a "catch-22" situation.
My understanding: The president can not take away a person's rights, regardless of the circumstances or perceived need for danger avoidance (at least not under the constitutional law as it currently stands).
Now - if there ever was a case such as you described, then it's not clear how any authority would act. However, if these individuals acted inconsistently with existing laws, then there would be consequences - likely governmental officials would be convicted of various crimes, and the suspect would go free as any evidence must be thrown out.
Perhaps, if a situation like this arises, we'll see changes to our laws. But, until then, the laws stand. And we must act accordingly.
However, as I stated above, it would be exceedingly difficult to extract the needed information from the suspect by any means, if he is intent on his actions. After all - all he needs to do is remain silent for another few hours - then BOOM! So, even in this awful scenario, a better intelligence agency is the only real solution.
p.s. In response to Cabbie's post, the Supreme Court has already ruled that Due Process must be followed - even for suspected terrorists
http://www.worldpublicopinion.org/pipa/articles/btjusticehuman_rightsra/228.php?nid=&id=&pnt=228&lb=bthr