So it turns out that the Sea-Based X-Band Radar wasn’t used to track the Unha-2, according to the LA Times:

Officers at U.S. Forces Korea, Pacific Command in Hawaii, the North American Aerospace Defense Command in Colorado and at the Pentagon all tracked the missile as it launched and passed over Japan and remained at their station around the clock as analysts examined the initial data from the launch. Analysts at the CIA as well as the National Counterproliferation Center remained on duty during the declared launch window, officials said. However, U.S. officials did not use one of their most powerful radar systems, the sea-based X-band radar, which would have been capable of monitoring a broader area in greater detail. Officials did not say why the X-band radar was not used. But the X-band system is a key component of the U.S. missile-defense system and its use could have been seen by other countries as provocative.

I have a better suggestion as to why it wasn’t used: because it’s undergoing repairs.

Update: Geoff Forden discusses the Washington Times story on this subject. Noah Shachtman quotes Bob Gates as saying it that it would have been too expensive to deploy SBX.