A deterrent against nuclear terrorism?

Clearly, this has been discussed behind closed doors (or at least I hope so) for many years now, but the U.S. is finally going public with some highly relevant musings:

Every week, a group of experts from agencies around the government — including the C.I.A., the Pentagon, the F.B.I. and the Energy Department — meet to assess Washington’s progress toward solving a grim problem: if a terrorist set off a nuclear bomb in an American city, could the United States determine who detonated it and who provided the nuclear material?

A guy I know poses this problem a little more bluntly and pungently: "Say we wake up one morning and Houston's gone. Okay, what's our next move?"

U.S. Debates Deterrence For Nuclear Terrorism (New York Times, May 8, 2007)