Slightly-modified comment I wrote on Barry Brook's blog.
According to EL's EPA link , iodine-131 activities in milk were measured at 3.2 pCi/L in Arizona, 8.9 pCi/L in Arkansas, and 18 pCi/L in Hawaii; the EPA's MCL is 3 pCi/L , which was exceeded. But this MCL is for life-long exposure, whereas this 131I will decay in a few days; it is completely harmless, despite exceeding MCL. For example the FDA's derived intervention level for 131I in milk is 170 Bq/kg or 4,600 pCi/kg [2,3]; this is 250x higher than the highest activities detected.
EPA administrator Lisa Jackson testified about this today: she says the same thing I'm saying: 
Early precipitation samples collected by EPA indicated low levels of radioactivity. Given the sampling results in other environmental media, EPA expected to find very low levels of radiation in precipitation samples. Similar findings are to be expected in the coming weeks as radioactive materials are dispersed through the air from Japan. While the levels in some of the rainwater exceed the applicable Maximum Contaminant Level (MCL) of 3 pCi/L for drinking water, it is important to note that the corresponding MCL for iodine-131 was calculated based on long-term chronic exposures over the course of a lifetime 70 years. The levels seen in rainwater are expected to be relatively short in duration and are not expected to present any threat to public health.
Results from samples of milk taken March 28, 2011 in Phoenix, Arizona and Los Angeles, California showed approximately 3 pCi/L of iodine-131, which is more than 1,500 times lower than the Derived Intervention Level set by the U.S. Food and Drug Administration. These types of findings are to be expected in the coming days and are far below levels of public health concern, including for infants and children. Iodine-131 has a very short half-life of approximately eight days, and the level detected in milk and milk products is, therefore, expected to drop relatively quickly. Additional information about the broader federal response can be found at: http://www.usa.gov/Japan2011.
Take the whole-body CEDE of 131I by ingestion.  (Disclaimer, I am not a health physicist). At a CEDE of 1.44*10^-8 Sv/Bq, drinking 1 L/day of MCL 3 pCi/L water gives you an accumulated dose of 40 μSv or 4 mrem (the base regulatory definition ) after 70 years. (This is, of course, nothing. You get 17,000 mrem (17 rem) of natural radiation in the same time period). If instead you have a one-shot 131I release, which gives you an initial 131I level of 3 pCi/L which then decays, you ingest a total of 1 L/day * 3 pCi/L * (8.0 days / ln(2)) = 34.7 pCi, for a total dose of 18.4 nSv or 1.84 μrem. This is ridiculously negligible. With the Hilo measurement, 18 pCi/L, this goes up to 110 nSv or 11 μrem.
Now, for contrast; in Fukushima prefecture dairy farms, milk 131I levels had been measured  up to 1,510 Bq/kg (L), or 40,810 pCi/L. Under the same assumptions as above, if you were to drink 1 L/day of this milk (which you couldn't been it was not sold), you could get a total dose of 250 μSv (25 mrem), over the few weeks it would take for the iodine to decay. Four orders of magnitude higher than in the US (but still not really harmful).