I think EMP has become such an issue (at least for me) is that terrorist groups are continually trying to make or obtain a nuke. If a terrorist group is able to obtain a single nuclear weapon then the way for them to get the most bang for their buck is employing it in such a manner to use the EMP to the utmost. I don’t think it will be knocking out the entire US because of the logistics involved in making a rocket large enough to put it high enough to cause that much damage. But with a small pressurized plane they could get high enough to cause a black out along the east coast that would knock out most of the electrical grid, causing blackouts across most of the country. Most of the country would be up and running again in a week or two but the damage to the east coast would take years to recover from. As far as nature we are constantly being bombarded by the radiation from solar flares and it constantly causing problems with satellites and communications. With every solar flare (that comes our way) we run a 50/50 chance of EMP doing a great deal of damage depending on it’s magnetic polarity.