I've seen a few articles on this recently, but this seems to be a main source.

Now, this is Worldnet Daily, so I am not going to lose any sleep about an imminent Iranian attack, but the principles and numbers laid out here are real, as are the effects of EMP.

I could easily see this as a way for terrorists to get maximum use from a nuclear weapon. A small or badly-designed nuke in an inhabited area would cause damage somewhat greater than the 9/11 attack. But a properly-done EMP pulse could take out a lot of equipment across a whole seaboard.

This link, also found in the article, has more information.

The interesting thing about this kind of attack is that it requires a great altitude of detonation to work over a large area. A missile is needed. This kind of attack cannot be carried out by a "smuggled" weapon in some harbor. This is actually good news, since it makes the attack harder for a terror organization in some ways.

While I suspect military equipment has been designed with this in mind, I have no idea if anything has been done about civilian infrastructure. When Y2K became an issue, a lot was done to fix things prior to the occurrence, but the kind of work that would need to be done to make the U.S. EMP-resistant is orders of magnitude harder than simple software fixes.

The terror potential would be a huge economic impact, with remarkably few lives lost. This may not appeal to most terrorists, who like a lot of death. But smarter terrorists may see some advantage in creating chaos without casualties. For one thing, it would make conventional nuclear retaliation (hey, they fired one off first), i.e. dropping a bomb on some Iranian city, much less politically feasible. Kill tens of thousands of civilians overseas because our lights and computers went out? Corrolary deaths, like those from the heat wave in Europe, would not be as dramatic.