It seems that the carry-on policy may vary according to whether the flight originates within the US (or a protectorate) or in another "untrustworthy" country. I've taken two flights since the underwear event. One was a LAN flight from JFK to Toronto. It originated in Santiago, Chile, on the 28th Dec. Things were still very much in flux. Normal carry-on luggage was allowed, but passengers were prevented (without any forewarning) from using the toilets over American air space. Created a bit of hardship on the pregnant lady sitting behind me.
The return flight was on American Eagle. For those of you who don't know, flights from Toronto to the US go through customs in Toronto. So, there are multiple levels of extras before getting to boarding area. Nevertheless, two new twists were added. The first was the presence of an employee in the check-in area whose job it was to determine what was and what was not allowed to be carried on the plane. Typical handbags and computer bags were allowed. A few small backpacks serving as handbags were also allowed. One of those three items per person. No exceptions. My typical "carry-on" had to be checked.
The second twist was an additional security check just before entry to the pre-boarding area. This involved a full body pat down, except for the crotch. Male patters for men; female for women. Carry-on bags were opened and searched carefully.
So, what does that portend for upcoming trips to the Caribbean? I am expecting that I will be able use a normal carry-on without checking it from the US to a final destination. Returning, I am not sure what will happen. We will be returning from Dominica on American Eagle and I don't know whether bags will be checked through to the final destination at JFK or if I will even be able take my carry-on as a carry-on. The added complication is that American Eagle planes typically require carry-ons to be given to baggage handlers on the tarmac to be stowed in the luggage compartment because there is insufficient overhead storage space. For fragile things, this is still better than putting them through as checked luggage. I suspect we'll learn more about this as we proceed from San Juan to Dominica, but at the moment, I am not certain how to pack for the return flights.