If you ever get asked that question at the US border, please don't acquiesce to that request. They have the right to ask, and they even have the power to search it regardless of your permission, but despite an alarming drift towards a total surveillance, they have not established the right to force you to unlock/decrypt anything.
I'm flying into SFO tomorrow, and I am taking similar precautions as chx so that my laptop doesn't contain any meaningful data[1].
However, if asked to unlock my laptop, I plan to say "No, of course I cannot do that; it violates the most basic security practices and I could and should be fired if I exposed sensitive company data in that manner." And then just sticking with it. It will be inconvenient, especially if they seize my laptop and detain me, but as citizens it is up to us to resist the normalization of behaviors that push the nation further towards the precipice of idiotism.
[1]: As an American citizen, I have routinely done this when traveling to authoritarian nations like China; it's hard to express how outraged I am that my own country has degenerated to the point where sound security practices now require these kinds of procedures when traveling to the USA.