I find myself very conflicted about this. I think the wording of "kill switch" is an extremely poor choice so I will try to illustrate what I think the genuine national security reasoning is, using China as the hypothetical aggressor. The Chinese government controls all of the hubs which connect the internet in China to the rest of the world, the "great firewall" as some have called it. The chinese use this for censorship, but it also puts them in an advantageous situation during a conflict. Imagine the impact widespread denial of service attacks on financial and governmental institutions would have on a nations ability to wage war (read estonia). China could launch such attacks while effectively defending itself against similar attacks from foreigners. The chinese subset of the internet would continue to function while blocked off from the rest of the world.

With that in mind it seems like the "kill switch" should kill or limit connections to outside the United States. That said, the power this would give the domestic leadership may pose a greater threat to liberty than a foreign aggressor. I think I agree with philwelch.. I would rather see the plans to do this confidentially drawn up by the CIA than written into law.