The government has now published its policy on laptop searches here. It raises more questions than it answers. For one thing, they don't just claim the right to search — and seize — your laptop when you enter the country; they can search it when you leave the country, too. They also claim the right to do this at the "functional equivalent of the border, or extended border". Declan McCullagh explained these and related issues. He also points out that CBP is enforcing trademark and copyright laws, which (at least in theory) gives them the right to look for illegally-copied songs on your iPod.
Peter Swire, a respected law professor and former Clinton administration official, has written on the subject as well. In his Congressional testimony, he, too, points out the similarity of laptop searches to cryptographic key escrow.