I don't there are many people out there who haven't heard about the fire at one of the nuclear facilities caused by the 9.0 magnitude earthquake in Japan. As of right now, there is a no-fly zone in parts of Japan because of this.
The winds currently are blowing out of the west and out to sea (see Al Roker). As long as this continues people not in the immediate vicinity of the facility will avoid the worst of exposure.
But what about flights coming from the U.S.? Well, that's where the problem comes in. In order to get to Tokyo from the U.S., flights will fly over the far northern part of the Pacific or Alaska. This is because the flight path is shorter when flights move toward the poles. For a long flight like NYC to Tokyo, this will mean flying near the northeastern part of Japan, right where the radiation cloud would be. While it might not be as bad at higher elevations, people will be careful. As of right now, the no-fly zone is about 18 miles around the facility, which most flights should be able to avoid. If the winds shift or if it gets worse, this will obviously change.