I dont think they will use it against a city.
See, if they nuke a city they KNOW the entire fargin world will come out after them and anhiliate them.
They know a nuked city would mean the US and every world power and organization (British Commonwealth, China, Russia, India, Pakistan, NATO, *sigh* UN, EU) and every arab nation that will be scrambling to offer their help in destroying those responsible to avoid getting invaded and annexed (oh yeah a nuke throws off all rules) would be coming after their hides with a vengeance.
No, they wont nuke a city to cause massive deaths. Remember 9/11 happened in the WTC and Pentagon because it was a symbol and it would create the biggest shock... they couldve just as easily thrown airliners against places who wouldve had a buttload more people in them.
If they use a nuke it will be against a symbolic landmark that would cause little casualties (little is relative..its a nuke we talking about). So id say mt rushmore or a civil war fort ..anything 'american' thats a good distance from a big populated area.
or against a military target of both historic and economic value.. like someone said above, nuke fort knox and you take out a tank school and gold reserve.