The 190-Million Person Exception to the Fourth Amendment
In the 1976 case U.S. v. Martinez-Fuerte, the U.S. Supreme Court ruled that contra the Fourth Amendment, the government can set up roadblock checkpoints within 100 miles of the nation's borders in order to check for illegal immigrants and smuggling. The Court ruled that if the stops are brief, limited to that purpose, and not fishing expeditions, the minimal invasion to personal privacy is outweighed by the government's interest in protecting the border.
The ACLU says that since September 11, 2001, the government has been steadily stretching the limits of Martinez, to the point where the Department of Homeland Security is using that case and the terrorism threat to conduct more thorough, more invasive searches at dozens of checkpoints across the country. With 33 checkpoints now in operation, we're not exactly to the point of "Ihre Papiere, bitte" Berlin yet, but the ACLU does warn that the area of the country 100 miles from every border and coastline would include about 190 million people, or nearly two-thirds of the U.S. population (see map below).
Moreover, post-9/11, the courts have been pretty deferential to increasingly invasive searches the government says are necessary for national security purposes. For example, federal courts have given the okay to airport seizures and thorough searches of laptops and other electronic devices belonging to people returning from abroad. Such searches can be conducted with no individualized suspicion at all. Some of those subjected to them have said it took weeks for the government to return their computers.
Should the courts uphold these increasingly invasive "border searches" under some vague national security exception, I don't think it's too much of an exaggeration to say that the Fourth Amendment would be close to non-existent for a large portion of the country.