Here is a diagram of the anticipated trajectory of the radioactive cloud.
And for Alaskans here is the pertinent information.
There will be some radiation from Japan lofted to high altitudes today by the low pressure system affecting the region, and if the radiation manages to escape being rained out, it could potentially be transported thousands of miles over the next week. A run of the HYSPLIT model following the path of a radioactive cloud emitted at 12 UTC (8am EDT) this morning shows the radioactivity being lofted 4 - 5 km in altitude and being transported over Alaska over the coming week. After a week of transport, this cloud will be considerably diluted, and I strongly doubt the radioactivity would be harmful to human health if rain or snow were to carry it to the ground over Alaska or Canada, assuming that the radiation levels currently being advertised at ground level in Japan are correct.
I am choosing to be comforted by Doctor Master's assurances, but I would personally feel much safer without the "assuming that the radiation levels currently being advertised at ground level in Japan are correct" qualifier. I am not saying I don't trust the Japanese exactly, it is just that they have demonstrated a desire to downplay the potential danger since the beginning of this catastrophe, and that makes a me a little uneasy.
I will feel tons better once scientists on the ground in Alaska test the air and determine whether it presents a legitimate danger or not. And let me just go on record as saying that nuclear energy has just become my LEAST favorite alternative to fossil fuels. Just in case anybody is taking a survey.