As a former member of the "ABC-Abwehrkommando" (NBC defense squad) of our military, there actually IS such a thing as a harmless dose and also quite harmless radiation. Alpha and Beta radiation, for example, are quite harmless, can be blocked by a very thin barrier (Alpha radiation can be blocked by tin foil, Beta by normal glass) and are also used medically, (chemo treatment) they don't pose a real threat as long as you aren't exposed to it 24/7. (duh) Gamma radiation, however, is quite a bitch and needs at least about 1,5 half meters of lead in it's way to prevent it from spreading, if I recall correctly. But there still is a harmless dose of Gamma radiation, which is quite low for humans, which also, of course, depends on your age and overall health. The meanest thing about radiation is, that you won't necessarily feel anything for a few years. (unless you are reeeaaally near a radiation source, of course) However, an overdose of radiation can and will raise the possibility of an outbreak of cancer and cause changes in your blood.

I'm more worried about the future of Japan and those people involved, as they will face a possibly big rise in cancer patients.... I'm not sure, though, the infos we're getting of Japan are still very vague and inconsistent. Let's just hope for the best and donate as much as possible.