Originally Posted by polonius

Mostly a good summary, but:

1) you cannot be forced to answer questions, and most certainly the 5th amendment doesn't get suspended just because you are at a border. But again, if you do choose to answer, you can be penalised for not answering truthfully

2) although "probable cause" is not required for "inspection," it IS required for a search. What the difference is between "inspection" and a "search" has been established by a number of court rulings, including U.S. v Arnold, which clearly established that looking into a computer is a "search". That same ruling also established that finding images of nude adult females (which are of course perfectly legal) did not provide "probable cause" to believe that there may be illegal pictures of nude female children. The ruling can be found here.

The important legal principle to understand is that although there is an explicit exception to 4th amendment protections at the border, the exception was created for purposes of controlling who is admitted, for preventing the entry of contraband, and to ensure that goverment revenues due for tariffs and duties are collected. The exception was NOT created to give the government free rein to have a poke around in anything they feel like just because it's a border, even if the FBI, DEA and other agencies have been somewhat successful at exploiting this hole in 4th amendment protection for such purposes. I was once referred for secondary inspection and had an inspector start rifling through a stack of credit and membership cards that was in my briefcase -- I objected successfully to this attempt at searching through these cards because there was no possibility that any of them could have contraband or an undeclared import and therefore they had zero reason to be looking through them.