I believe I read the full explanation in the Yahoo Iodine Group, the short version is that iodine is an oxidizer and vit. C is an anti-oxidant and one hinders the other (I think the iodine destroys the vit. C...) so both should not be taken together.
In reality, this is not unusual at all when it comes to taking nutrients, a similar thing happens with iron and vit. E as iron destroys vit. E. There are also other supplements whose absorption is also optimized if not taken at the same time, such as calcium and magnesium which, despite the many formulas that include them together, are supposedly best absorbed when taken separately: magnesium in the morning and calcium at night. I suppose the form of these two minerals may also have something to do with it but I usually take them separately just in case; also it was recommended to me take magnesium tablets on an empty stomach to maximize absorption.