Considering how many of us own pocket computers that pass as cell phones and some of us take them while riding, this is a very important thing to remember. You never know if you have content that may be illegal (hell, an mp3 could land you in hot water if the DHS guy is being a dick), so it's better to be safe than sorry. They have so many laws on the books that they can't keep count. While this article is more from a legal professional's standpoint, it applies to any normal citizen.
http://www.cba.org/cba/practicelink/tayp/laptopborder.aspx
http://www.cba.org/cba/practicelink/tayp/laptopborder.aspx
Also remember that even a minor traffic infraction can be considered "an arrestable offense" and the cop can search your personal data (even though you're not required to give him the password to access your computer/cell). Read this article and keep it in mind next time you wanna hit the GapDo you regularly travel to the U.S. on business? If you take confidential information of any kind with you, take heed: US policy allows offers of Customs and Border Protection (CBP) to search and confiscate computers, phones, personal digital assistants, cameras, digital music players and other data-storing devices. Operating under the U.S. Policy Regarding Border Search of Information, agents have also downloaded the contents of entire computer hard drives and other storage media for later review. (Note: similar situations occur at the borders of other countries as well.)