Any US Customs entry point. Airports or land border crossings. Slashdot regularly posts horror tales about this. Some links relevant to my post.
Note that most of what I have read applies to two categories of people:
Those of the “wrong” ethnic/religious/national origin group.
People who have gotten the attention of the US government for espousing the “wrong” view on issues, such as laptop seizures at borders.
Note that you don’t actually have to crossing the border to be stopped and undergo an inspection by US Border Patrol agents. You can be up to 100 miles from the Canadian border.
I don’t have a lot to add to what has already been said. Someone examining your hard drive will take a forensic image of the drive, make copies, and work off the copies, so the killword won’t do much to affect them (I am a computer forensic examiner). Also, as has been noted, wiping a drive takes a loooong time.
Even if the killword approach did work, and you could somehow wipe a drive in a short period of time, you might not like the results. If the bad guys are hitting you with a rubber hose to get the password, and you trick them into wiping the data instead, things will probably go downhill for you pretty rapidly.
What you want is Deniable Encryption, as si_blakely noted. Interestingly, one early form of deniable encryption was called Rubberhose, having been designed for dissidents and reporters working in repressive regimes. The idea is, you give the Rubberhose Wielder the password to the innocuous stuff, and he never knows of the existence of the good stuff. One of the three developers of that system was … Julian Assange.
Rubberhose is no longer being maintained, and I would hesitate to rely on it myself, as I don’t have any information on whether the system has stood up to cryptanalysis in the decade or so since it was released.