Assuming it’s a high altitude burst (for example 250 miles above Des Moines, Iowa, so as to take out virtually the entire grid in one burst), I suspect it would indeed be seen. The few space test explosions of nuclear weapons in the early days were quite colorful and spectacular. But heard? I assume not, because there would be insignificant atmosphere up there to cause the usual pressure wave that we’d experience as movement of our ear drums. Even on earth, the largest nuclear blasts would not be heard hundreds of miles away, most likely – that that would require full atmospheric pressure for even that to occur.
I’m with MountainBiker on the mechanism, though. Short of Little Kim managing to convince his generals to actually launch something like that against the US, I don’t even think Iran is stupid enough to actually destroy all useful equipment, nation wide, when re-supply of the continent is automatically difficult because of thousands of miles of ocean on both sides. Only if an enemy wasn’t willing to come in and exploit this nation, but was willing to make it an instant wasteland for decades if not centuries, would I expect it to be from one or two large, high altitude bursts. If used as a tactic (and I think it’s quite plausible), I also suspect it would come by other means, as MB said.