According to AP, the Hermit Kingdom "has stopped disabling its nuclear reactor and will consider restoring the plutonium-producing facility in anger over Washington's failure to remove it from the U.S. list of terror sponsors."
The North Korean regime may also be annoyed that China's President Hu Jintao had just met with President Lee Myung-bak of South Korea. Or it may want some of the limelight of the US election coverage. Hard to know for sure.
Interesting to note the varying weight different news agencies give to the North Korean claim that the US had "violated" the agreement. For example, Reuters (via IHT) buries the specifics of the North Korean accusation, but Pyongyang's specific accusation features prominently in stories published by the Seoul Times and AP. According to AP "Removal from the terror list is one of key concessions offered to the North in exchange for shutting down and disabling the reactor under a landmark six-nation deal reached last year."
Update: Perhaps with all the acrimony exhibited by Washington towards Moscow, the North Koreans are testing the waters. There are too many issues -- like North Korea -- where the Russians and the Americans simply must work together.