One issue that has been overlooked a bit in the post-Sept. 11th world of foreign affairs law is whether the executive branch possesses a "foreign affairs power" to the exclusion of other governmental entities. This issue was famously debated between Jack Goldsmith and Harold Koh in dueling Harvard Law Review articles in 1998. At the time, the focus was on whether state's had any foreign affairs powers themselves and how the debate impacted court application of customary international law. The law that sparked the controversy was the Massachusetts Burma Law, which imposed a state sanctions regime against Myanamar.
In essence, Goldsmith and Bradley outline the limits of the foreign affairs power and argue that there is strong evidence that states have retained some rights to engage in limited foreign affairs actions, in particular, actions that augment or supplement federal foreign affairs activities. The U.S. Supreme Court's decision in the case does not answer the question, but implies that as long as state actions with international consequences are not preempted by a federal scheme of sanctions, they could be valid (a blow to the exlusive foreign affairs idea).
In an era of substantial political differences among states, it seems that a bold maneuver for states to affect the international stage is bound to emerge. It isn't difficult to imagine a world in which New York passes a law that aims to impact the Middle East Peace process, or to reward other nations with favorable policies with potentially large state contracts. Incorporating state lawmakers in the gains and publicity of international affairs could also encourage them to feel a greater stake in ensuring the success in overcoming huge obstacles facing bureaucratic cooperation.
Advocates of federalism should embrace this idea as states work to both make the war on terrorism more effective and more accurately reflect the wishes of their population. Don't get me wrong. Allowing states a piece of the stage of world politics is a bit scary. State governments are more likely than the federal government to act on the fringes of the political spectrum, as long as they are bound by actually executive order / legislated foreign policy, they can serve to make the law more clear and address areas that the federal government is unwilling to address for whatever reason.