As Mike Riggs notes below, the Department of Homeland Security now claims the right to seize your laptop "absent individualized suspicion" for as long as it deems "reasonable" whenever you dare cross an international border. Back in June, Ohio State University Law Professor Peter Swire slowly explained (pdf) the stupidity of this approach to the Senate Committee on the Judiciary. First, the futility:
Laptop searches will not succeed at a technical level at preventing data from entering or leaving the United States. Computer security researcher Chris Soghoian in May posted a story called "Keep Your Data Safe at the Border. Soghoian presents an eight-point checklist for how to get your data legally across the border without being searched. The primary trick is to send encrypted files to yourself once you get to your destination country.
The Soghoian article shows the futility yet burden imposed by laptop searches at the border. Any terrorist who is even moderately well-informed can learn how to send the crucial files legally and safely across the border. In addition, a terrorist who is willing to lie to the customs agent (certainly a possibility worth considering) can use TrueCrypt or other software that does the following trick—it allows you to encrypt a secret cache of data inside your encrypted hard drive. Then, when an investigator forces you to open your encrypted files, the secret cache remains invisible to the investigator. This TrueCrypt approach requires lying to the custom agent about whether you have opened up all of your files, but it is a technical measure already available with widely available software.
The evildoers wouldn't lie to a customs agent, would they? It's always a good sign when your strategy relies on the moral integrity of terrorists. Swire's testimony also includes a list of the kind of material one might not want to share with some guy at the airport: "diaries, love letters, a lifetime of saved email, private photos, passwords, financial and medical records," trade secrets, campaign secrets, journalists' notes and so on.
As commenter "nothinghead" (not a helpful handle, but hey) mentions below, visitors headed to China are being advised to either encrypt or wipe data before flying out. Sam Brownback (R—Kan.) complains that Americans "will be subjected to invasive intelligence-gathering" by China's Public Security Bureau. Imagine that.