In the unlikely event that one of the Japanese nuclear power cores melts down AND breaks through its containment vessel, you can check for real time data on radiation levels in the United States, here.
Note: Overlaid on top of the main graph for the local Monitoring Station is a National Radiation Map, where each colored circle represents a Radiation Monitoring Station at that location on the Map. The numerical value displayed in each circle represents the Radiation Counts Per Minute (CPM) for that station during the last minute.
Typical background radiation levels for most of the USA are in the 5 to 18 uR/hr range. Readings can be higher for brief periods of time due to normal variations in radiation levels. They can also be consistently higher for areas at high elevations, or with larger natural deposits of uranium, thorium, radon, etc.
Caution the page is still partially under construction and certain readings are not in real time. I am attempting to identify which readings are not in real time and will add that information as an update to this post.
Update: Here's what I am getting from the people who constructed the map. The readings are being sent by individuals across the country who have hooked up their radar detection equipment. Some may no longer have it connected. All detectors have slight changes in readings over time, so if you don't see any changes over a few days, assume that the detection equipment is no longer sending up to the minute readings.
Update 2: After monitoring the site for a few days, it appears that all data provided on the site is now real time data. The sites that weren't updating seem to have all been removed.