Similar to the China (or maybe Russia) theory. Simply put, a rogue nuclear power decides to slip several bombs into our country in an attempt to reduce American power over other countries. This creates a power vacuum, which can be exploited by the bombing country. China, while in business with the US, might have reason to fear American military domination, or benefit from removing the US from superpower status (Pax Sinica!). North Korea is more of a long shot, but Kim Il Sung and Kim Jong Il both railed against the USA and its "imperialism". (Thus, a successful nuclear terrorist attack would prove a valid rallying point for the powers in Pyongyang.) Iran, which hates Israel and thus hates its ally America, would have similar motives and similar goals.