There’s bad news out of Japan about high levels of iodine-131:
the level of radioactive iodine in water at the plant hit levels 10,000 times the permissible limit, preventing workers from getting near the water
Iodine 129 (half-life 16 million years) might also be of concern, because, as a fact sheet (pdf file here) out of Argonne National Labs states:
The fission yield of iodine-129 is about 1% and the yield of iodine-131 is close to 3%. That is, about one atom of iodine-129 and three atoms of iodine 131 are produced per 100 fissions. Iodine-129 is present in spent nuclear fuel, …
The long half-life of I-129 means that it decays very slowly, such that emissions from I-129 should be of far less concern than I-131, which has a half-life of 8 days. Still, if there’s a lot of I-129, it could still be problematic for health and the environment. So, it will help to analyze for I-129 here also.
Science wonks might also note that the long half-life of I-129 ought to help facilitate tracking contamination in the sea and in the subsurface.