Warrantless Searches Happen All the Time
Former federal prosecutor Andrew C. McCarthy provides a list of 28 exceptions to the 4th Amendment’s requirement that a warrant be issued prior to a search. The list is a bit exaggerated by multiple listing of public safety exceptions but his point is nonetheless a good one.
Of course, “electronic surveillance of Americans suspected of terrorist ties when at least one party is overseas” is not among the 28. Whether it will be deemed to be by the courts remains to be seen, although it is certainly less “unreasonable” than some of those already on the list.
Hat tip to commenter Mal.