From the Union of Concerned Scientists:
A significant amount of radiation has been released to the atmosphere from this site since the beginning of the crisis. Two of the main health hazards from the radioactive gases that have been released are from iodine-131 (I-131) and cesium-137 (Cs-137). One analysis estimated that roughly 20% of the I-131 and up to 50% of the Cs-137 released in the Chernobyl accident was released from Fukushima to the atmosphere within the first few days of the accident.
Very high radiation levels are being detected at some points many kilometers away from Fukushima, outside of the evacuation zone, although there is no clear picture at this point because the locations of the readings are not publicly available and there has not been a systematic survey.
As the radiation is carried by winds across the ocean, it spreads out and becomes diluted. While trace amounts have been detected in the US, these amounts have been much lower than the natural background levels of radiation that people are constantly exposed to, and are not a serious health hazard.
Daily updates from Japan:
Union of Concerned Scientists: