People in China, Hong Kong and Taiwan were "panic buying" salt. These places are so far that any radiation from Japan will diminish way before reaching them. This is another product of irresponsible reporting by mass media.
Anyway, why salt?
The salt thing began with some couch experts saying "taking additional Iodine can prevent radiation from affecting your Thyroid Gland (so that it won't store radioactive Iodine)." I don't think this is something an ordinary person can manage.
Iodine -> Iodized salt -> salt -> by all salt you can buy
My Chinese friends calling home have a hard time convincing their parents not to panic. Given the wind patterns, mountains and distances, any chance of other countries being affected is near zero.