They've been able to strike at both South Korea and Japan for quite a while now. Most of the reason we're worried is because the advent of No. Korean missles with extended range, such as the ones they're testing now, have the capability of reaching all the way across the Pacific, to hit Hawaii, Alaska, or maybe the western coast of the continental US.
Though I'd say that Alric's right. Capable or no, North Korea won't want to start a nuclear war. Let them arm all they want--they've got a distinct disadvantage if it comes down to shots being fired. Mostly they want the nukes as an international bartering chip. It shouldn't work, really, since if North Korea actually tried to use a nuke on anybody, China, the US, Russia and probably a few European nations would ALL have nukes sent inbound at North Korea. Korea isn't all that large a country--with that much firepower bearing down on it, there'd be nothing more than a crater left. Korea knows that. The rest of the world seems to be forgetting, though. The North Korea situation really oughta be considered a "Hey, so what?" type of deal. Same with Iran, honestly. Though I'd worry about Iran more, they might just have the juevo's to launch a nuke at Israel if they got the chance, even if it would be a sort of suicidal act.