The North is believed to be nearing completion of an electromagnetic pulse bomb that, if exploded 25 miles above ground would cause irreversible damage to electrical and electronic devices such as mobile phones, computers, radio and radar, experts say. _abcnews
Given the heavy dependency of modern societies on electric power, it has been estimated that long-term loss of electric power over a wide area could result in loss of up to 90% of pre-event population. Such a high mortality rate could occur due to a combination of long-term basic infrastructure breakdown combined with the inability to bring significant aid into the area from the outside.
Both the US and the USSR conducted experiments to determine the effects of EMP from high altitude nuclear detonations:
A few hours after the sun rose in Kazakhstan on [a] cloudy October morning, the Soviet Union detonated a 300 kiloton thermonuclear warhead in space at an altitude of 290 kilometers (about 180 miles) over a point just west of the city of Zhezkazgan in central Kazakhstan. The test was generally known only as Test 184 (although some Soviet documents refer to it as K-3). It knocked out a major 1000-kilometer (600-mile) underground power line running from Astana (then called Aqmola), the capital city of Kazakhstan, to the city of Almaty. Several fires were reported. In the city of Karagandy, the EMP started a fire in the city's electrical power plant, which was connected to the long underground power line.And those effects in Kazakhstan resulted from the detination of a bomb not particularly efficient at producing EMPs. A custom-designed EMP nuke would have done a far more thorough and devastating job of it.
The EMP also knocked out a major 570 kilometer long overhead telephone line by inducing currents of 1500 to 3400 amperes in the line. (The line was separated into several sub-lines connected by repeater stations.) There were numerous gas-filled overvoltage protectors and fuses along the telephone line. All of the overvoltage protectors fired, and all of the fuses on the line were blown. The EMP damaged radios at 600 kilometers (360 miles) from the test and knocked out a radar 1000 kilometers (600 miles) from the detonation. Some military diesel generators were also damaged. The repeated damage to diesel generators from the E1 component of the pulse after the series high-altitude tests was the most surprising aspect of the damage for the Soviet scientists. _FutureScience
It is not a question of "if," but of "when" and "where" such an attack will occur. If it occurs over a modern nation which has not prepared for it, the loss of life is likely to be severe. In addition, the target nation would temporarily lose its ability to operate effectively, internationally. The widely dispersed nature of the US suggests that that country would maintain significant retaliatory capacity, but the civilian infrastructure would require massive and lengthy re-building. During that time of re-building, it is likely that many millions of people would die from a wide range of causes caused ultimately by the attack.
Good introductory article on EMP from FutureScience
Wikipedia entry on EMP
Federation of American Scientists article on EMP
EMPact America EMP resource
"One Second After" website for novel based on EMP attack over most of US, and subsequent aftermath
If the damage to the electrical infrastructure from a coordinated EMP attack were widespread enough, restoration of widespread grid power could take years. Over that time period, many large cities would likely revert to barbarism without overwhelming military presence. With hundreds of cities involved, it is unlikely that the US military could preserve order in more than a few.
That is why any hint at development of EMP weapons by North Korea, Iran, Venezuela, etc. must be taken seriously. While it is true that any nation launching an obvious EMP attack against the US would probably be obliterated by retaliation, it is possible to launch a satellite containing an EMP weapon -- but with a different primary use. Such a "sleeper" satellite could orbit for years until it was felt strategically opportune, by its owner, to detonate it. By the time a case could be made against the true culprit, the US (or Europe etc) may no longer be functioning as a viable civilised society.
As you can imagine, preppers and survivalists have taken an interest in the EMP phenomenon, with an emphasis on how to prepare to survive such a widespread disaster. Here is one PDF essay on that topic from Prepper.info
You will need to use your imagination here. What would happen to your neighborhood, community, town, city, etc. if it were to lose power for several months, without significant outside assistance? How long would it take for the stores to run out of food and supplies, or for the fueling stations to run out of fuel? How would you feed yourself and your family, if civil disorder set in -- preventing a cooperative community effort to share resources?
Such weapons may be in orbit overhead, even now, waiting for a designated time to inflict chaos on a targeted continent. What should you do?