I strongly suspect the United States government is fed up with the lack of accurate information being provided by the Japanese government. This will give the American government an independent source of information with a powerful radar capable of seeing through the smoke and dust of the reactor site. These drones can stay in the air for almost 24 hours on one mission.
From the New York Times -
United States Air Force officials announced Wednesday that a Global Hawk remotely piloted surveillance plane would be sent on missions over Japan to help the government assess damage from the earthquake and the tsunami. A Pentagon official said the drone was expected to fly over the stricken nuclear plant.
From Wikipedia -
In role and operational design, the Global Hawk is similar to the Lockheed U-2, the venerable 1950s spy plane. It is a theater commander’s asset to provide a broad overview and systematic target surveillance. For this purpose, the Global Hawk is able to provide high resolution Synthetic Aperture Radar (SAR)—that can penetrate cloud-cover and sandstorms— and Electro-Optical/Infrared (EO/IR) imagery at long range with long loiter times over target areas. It can survey as much as 40,000 square miles (100,000 square kilometers) of terrain a day.
Missions for the Global Hawk cover the spectrum of intelligence collection capability to support forces in worldwide peace, crisis, and wartime operations. According to the Air Force, the capabilities of the aircraft allow more precise targeting of weapons and better protection of forces through superior surveillance capabilities.