Even the 'reasonable' searches require a warrant based on a sworn afidavit and probable cause.
Read the 4th Amendment. It won't take long.
I have many times and reasonable searchs don't require a warrant. That's why they search your baggage at customs without a warrant. It's considered reasonable. That's why customs can inspect incoming and outgoing shipments without a warrant and search ships. That's why we all get searched each and everytime we fly commerically. They are considered reasonable under current conditions. What's reasonable searching for a small amount of drugs (or a large amount for that matter) is very different than what's reasonable searching for a nuclear weapon that you think will go off shortly for example.
That's why the issue in terms of the needs of warrants not only revolves around the presidential foreign affairs and war powers but even if the Bush administration is wrong about those then were the actions reasonable under the circumstances at the time considering what the threat was. If searching each and every passanger flying commerically is reasonable due to the terrorist threat then wouldn't listening to and taking action on converstations between Al Queda operatvives overseas talking to people in the US also be reasonable? Considering what happened on 9/11; the first World Trade Center Bombing; the thwarted attack on the Brooklyn Bridge; the Cole Bombing; the thwarted attack on Los Angeles International Airport, etc. I think it is reasonable. Listening into someone from one of the drug cartels calling a major dealer in the US without a warrant would IMO be unreasonable. While both are threats of serious harm the first one is far more dangerous and represents the highest level of threat.
Here is some material on it:http://www.njsbf.com/njsbf/student/eagle/winter00-1.cfm
http://en.wikipedia.org/wiki/Fourth_Amendment_to_the_United_States_Constitution
http://www.nolo.com/article.cfm/catId/3900BEB2-2599-4E9F-B5F09F0DF3E33C7B/objectId/DED24689-ADA8-4785-887A0B4A19A694DE/104/143/ART/
http://www.oyez.org/oyez/portlet/directory/0/16/
http://www.lectlaw.com/def/f081.htm