...The G.A.O., the investigative arm of Congress, described in a 13-page report delivered to Congress Thursday how easily they were able to penetrate the border at several spots....
Here's the 27 Sep 07 GAO report:

Border Security: Security Vulnerabilities at Unmanned and Unmonitored U.S. Border Locations
....On the U.S.–Canada border, GAO found state roads close to the border that CBP did not appear to man or monitor. In some of these locations, the proximity of the road to the border allowed investigators to cross without being challenged by law enforcement, successfully simulating the cross-border movement of radioactive materials or other contraband into the United States from Canada. In one location on the northern border, the U.S. Border Patrol was alerted to GAO activities through the tip of an alert citizen. However, the responding U.S. Border Patrol agents were not able to locate GAO investigators. Also on the northern border, GAO investigators located several ports of entry that had posted daytime hours and were unmanned overnight......