I spoke with US Customs Chief Inspector Dan Sheehy about this.
He said:
1) If US Customs has probable cause to believe you are smuggling
contraband, then you will be closely searched. If you arrive wearing a
badge that says "International Encrypted Kiddie Porn Emporium" then they
may indeed seize your laptop and send it to the lab to see if they can find
any encrypted kiddie porn on it.
2) If you do not give them probable cause, they will not search your
laptop. If you declare it to be a laptop, and it looks like a laptop,
that's the end of it. Customs inspectors have no procedures and no
resources for scanning disk drives. There is no procedure for seizing
_anything_ without probable cause.
3) The foregoing applies to citizens and noncitizens equally.
