Nah, nothing to worry about. An Arliegh Burke-class destroyer joins the 7th Fleet. Whoopity-doo. Joining the how many already there?.
It was probably a routine redeployment, relieving another ship for rotation home, and the press picked up on it and decided to ramp up the fear factor in order to sell a few more papers/get airtime/ratings.
IMHO, the only way we're going to start shooting at North Korea, is if they start firing LIVE missiles at Japan/South Korea/us. As seajay pointed out, they already have nukes, and a pre-emptive strike would be a little too risky.
The Navies of any country don't move that fast, it must have been planned a while ago. Koreans are mad, and they like pissing people off, but I really don't think they are stupid enough to go to war with nukes............ I hope.....gulp 8O