The following is from a transcript (via Cryptome) of Roscoe Bartlett (R-MA) speaking about the threats posed by nuclear weapons being used to create EMP pulses:

Iran has conducted tests with its Shahab-3 missile that have been
described as failures by the Western media because the missiles did not
complete their ballistic trajectories, but were deliberately exploded
at high altitude. This, of course, would be exactly what you would want
to do if you were going to use an EMP weapon.

Today we are very much concerned, Mr. Speaker, about asymmetric
weapons. We are a big, powerful country. Nobody can contend with us
shoulder-to-shoulder, face-to-face. So all of our potential adversaries
are looking for what we refer to as asymmetric weapons. That is a
weapon that overcomes our superior capabilities. There is no asymmetric
weapon that has anywhere near the potential of EMP.

Iran described these tests as successful. We said they were a failure
because they blew up in flight. They described them as successful. Of
course, they would be, if Iran's intent was practicing for an EMP
attack.

Iran's Shahab-3 is a medium-range mobile missile that could be driven
on to a freighter and transported to a point near the United States for
an EMP attack. I might state that an early use of EMP is a common
occurrence in Russia and Chinese war games.

The DPRK is also an entity we would have to worry about attempting an EMP attack, although the likely target would be Japan.

I found this speech very interesting. I have thought about EMP before, but I only considered it a threat that would come from the more advanced nuclear powers, such as Russia and China. Bartlett makes it expressly sound like that is not the case. I decided to do a little research, so I started flipping through the Industrial Memetics rolodex.

After speaking to a friend who is an expert in Nuclear Physics, it doesn't look like this is something we would have to worry about coming from Iran, DPRK, or Al-Queda. In the situation Bartlett used as an example, a nuclear weapon detonated 400-600km over the United States, the EMP charge released would be measured in the millivolts-per-meter range, assuming the weapon had a yield of around 30 kilotons. That yield is a best guess for what we could expect of a first generation weapon from an entity like Iran, the DPRK, or Al-Queda. That's a far cry from the type of weapon that created the 5 kilovolts-per-meter pulse experienced in Hawaii during the Fishbowl Series of tests in 1962. Its safe to assume that Iran and the DPRK do not have tritium production and are not working with thermonuclear bomb designs yet.

I did not think to ask what the EMP strength would be like for a 30kt device detonated at 100km. Enough to take out civil communications/power somewhere like NYC or Washington? Given the discussion I had, I'm assuming the EPM created would still be of negligible strength. At that point, nukes start to look more useful in the context of traditional style tactical delivery.

Bartlett on EMP