http://www.nuc.berkeley.edu/UCBAirSampling

So far, levels of radioactivity are increased but not in any danger zone. If you're concerned about radioactivity in the US, UC Berkeley posts its monitoring results at least daily, sometimes more often.

For links to sites with information on radioactivity, how it's measured, and how to judge the potential damage caused by exposure to radiation, browse the links at http://civex.posterous.com