While not a doctor, I can confidently state there is no reason for residents of the U.S. to purchase iodide tablets unless you live in the immediate area of a nuclear facility, the U.S. Surgeon General's comments today not withstanding. There are people who are allergic to iodine (and most don't know it) so there is very much a downside to taking them when it is not necessary. If you live near a nuclear facility (within ten miles), it might not be a bad idea to keep them in case of an emergency but they are not needed now.
I cannot conceive of a situation where sufficient radiation from Japan could cause a problem in the contiguous United States.