When you say full blown nuclear war what are thinking of? If you are thinking of the type of nuclear war we used to imagine when the Soviet Union and US had the Cold War going - I don't think so, at least not until the world takes a more serious turn for the worse.
If you are talking about some group - or government - detonating nuclear warheads in US cities - or nuclear dirty bombs in US cities - then I think that is possible - but I honestly don't know what the likelihood is.
I think it is entirely likely that either terrorist groups - or governments that are not our friends - possess nuclear weapons. From all of the evidence North Korea has detonated a nuclear weapon. We also know that North Korea always needs money - Arab oil states possess copious amounts of money - do the math. I have also read that when the Soviet Union broke up nuclear weapons went missing. By some of the reports I have read the missing Soviet nukes were suitcase bombs - but they need maintainence - so they may not work any more. From everything I have read it is entirely possible that a nuclear dirty bomb could be constructed. There is also a heavy Islamic presence in areas of Africa where uranium is mined - another way terrorists or governments might get hold of radioactive material.
I have also read numerous reports that persons of Arab and other non-Mexican heritage have been caught sneaking across the US-Mexico border. The reports I read said some of these people paid up to $50,000 to get smuggled into this country - again do the math. Why would you pay $50,000 to get smuggled into this country when the going rate for a Mexican is something like $1500 - $5000? Especially when you consider it would take quite a long time to make that money back if you were just coming here to work. You pay $50,000 when you positively don't want to get caught.
So you have the distinct possibility of nuclear weapons of some form existing in the hands of people who may want to use them on us - and the verified cases of persons of likely terrorist origin getting smuggled across our southern borders. Again - do the math.
The real question is WHY would some group or government do this on US soil? Terrorists like Al Quaeda might do it just to make a statement. Bin Laden has made statements something like an equal number of Americans must die to make up for the numbers of Muslims that have died. A nuke would be one effective way of doing that. A country like Iran might do it to us - but I personally do not believe that they would commit terrorist acts of this magnitude on US soil - unless we attacked them first. Which from all news accounts the US appears to be gearing up to do.
Everybody prepares for this differently - some people always have a survival pack with them or in their car - and they stockpile a certain amount of food and water to ride out a couple week period when everything might be screwed up. Other people stick their head in the sand and say it can never happen here. I think a lot of this depends on where you physically live. If you live in a major city you are much more likely to have something like this happen to you than somebody who lives out in the plains of South Dakota.
I live about 20 miles outside of Boston - which is in my opinion probably on the short list of places where something like this might happen. My biggest fear is not getting killed by a nuke or a dirty bomb - but the after effects of that. Since terrorists would likely be detonating a nuke or dirty bomb on the ground I probably would not be exposed to any blast effects this far out. Radiation is a worry - but I think the prevailing winds would carry most of it out to sea. In my opinion the biggest worry would be the massive amounts of people who would be in exodus out of the city. Massachusetts is a pretty liberal state - and Boston is particularly so. You can bet most of these people are completely ignorant of keeping themselves alive outside of a city environment. The way I look at it my biggest worry in the end is protecting my own property and any supplies I have from being looted by those who have not bothered to think ahead at all. Like what happened in New Orleans you can bet that police and fire would be totally overwhelmed and for things like stopping looters I would be on my own.
For a few years after 9/11 I did not worry about this so much - the more and more I have paid attention to the Mideast situation the more I think this is possible now though. I got fully licensed in MA for firearms carry and possesion last year - and am slowly working on building up my firearms inventory with what I think is appropriate. I have also tried convincing my wife that we need to be more prepared but she just doesn't have the same level of concern I do - so I can't say we are fully prepared as of yet although we could probably ride out a week or so with what we have on hand.
If you search online you will find quite a lot about this subject. You can get radiation detectors, water purifiers, medical kits, survival guns, etc. It all makes for interesting reading although you have to be careful about dropping into paranoia sometimes.
Out of curiosity what was the other forum you were reading where people were discussing this? I am always curious to read what people are saying about this kind of stuff.