Via TravelTech Talk, the Electronic Frontier Foundation has tips on securing your data from the prying eyes of U.S. Customs when crossing the border into the US.
In short, unless you’re aware of a pending lawsuit or criminal investigation, you have no obligation to preserve data. And it’s perfectly fine to erase or encrypt it. They’ve got some links to help you, since the US government now claims the right to search your data based solely on your entering the country.