A weapons engineer physicist from Sandia Labs is working on a radiation detector ($12,000 and counting) that can be mounted on a pilot boat that goes out to meet the ship at sea, or we can build detectors on land that check the containers as they are driven out of the port, can be driven around, and cost upwards of $600,000.
http://www.wired.com/news/technology/security/0,71632-0.html
http://www.wired.com/news/technology/security/0,71632-0.html