If I remember correctly from childhood (I was chemistry-geek six year-old when Chernobyl blew up and the radioactive cloud drifted right where we were living at the time, but there were lots of talks about it later on), the idea is to saturate your thyroid gland with non-radioactive iodine, so that it won't absorb the nastier kind. It offers no relief from other types of radiation.

Problem is that your liver and kidneys are working hard to remove excess iodine, so the tablets only give about 1 day protection. (mind you, I think the half life of radioactive iodine is 8 days, so a few weeks course might be enough).

And not to scare-monger, but I remember my teachers saying that radioactive iodine is the most lethal type of radiation (not instantly, but it is highly carcinogenic = thyroid cancer later or).

-jh