The Environmental Protection Agency has up an excellent page tracking radiation levels in the United States. No elevated levels are expected from Japan and the page confirms this. Here is the latest update:
March 19, 2011
As of 8:30am (EDT) EPA's RadNet radiation air monitors across the U.S. show typical fluctuations in background radiation levels. The levels detected are far below levels of concern.
A good page to bookmark if this interests you.
And since California has been of particular interest, here is the latest report from the South Coast Air Quality Management District – this is from yesterday:
As of today, radiation levels measured at three regional sites operated by the South Coast Air Quality Management District for the U.S. Environmental Protection Agency have not been higher than typical “background” levels seen before the earthquake and tsunami in Japan. In the unlikely event that this changes, these extremely sensitive monitors will detect any change in outdoor radiation levels and it will be reported on this website.
It also has up a video with Dr. Jonathan Fielding discussing radiation issues. EPA uses reports from this and other air quality stations to compile its own.