Radiation wouldn't really affect our actual crops growing in north american. Radation from Chernobyl
hit the CA coast about 10 days after explosion and although you could measure it, it wasn't powerful enough to cause any issues.
Japan is 5,500 of miles away, regarless of radiation output in Japan, it's not going to affect north america's growing crops. (For compairson, distance to Chernobyl from LA is 6,300 miles.)
I've been reading up on Chernobyl, as many others have these past few days, and the radation danger is a local issue. Sure farms within 500 miles are going to be more suspect, but there is nothing in the Pacific ocean in that range, other than the actual island of Japan and winds will blow radation out to sea, and not blow towards Russia/ China, etc.
The biggest issue those of us in the states will have to deal with is inflation in prices. Food inflation is already rampant, huge increase have already been seen, and it's climbing higher everyday. Expect great inflation in speciality foods like sushi prices/ fresh fish prices, etc. Just in the last month, food inflation is almost 4%... the biggest increase in prices since 1974
Here is a current article just today about food inflation http://www.commercialappeal.com/news/2011/mar/17/food-inflation/
Japan already imports more than 50% of their food, but have feeling that percentage will rise as a result of this disaster. So cost of food might go up even higher, just because of increase in demand, but food infaltion is already an issue here, just expect it to get worse.
So go ahead, stock up on some things now, but do so from a cost perspective and not from a fear of unafe food perspective.