I have to assume they would eventually. Magnets and screens DO OT MIX, EVER! lol
I actively avoided them on principal and I see no reason why you should chance ruining an expensive gadget over the case desing. Just get something with a velcro enclosure or a snap button is probbably the best answer.
I cannot say for sure whether they would ahrd it but just in general the ~1 rule is DONT PUT THEM NEAR MAGNETS OR IN WATER.lol Just be safe man cause I havent seen anyone give reliable answer to this question so far and I have googled around.
I have a case with a magnet snap strap thing. I have been carrying it in this case since I bought the ipad. I dont see any difference in wifi or 3g performance in or out of the case and I have had no problems so far.
There are very powerful magnets inside hard drives, right next to the magnetic media that is written on. Its all about where the field goes I guess.
Its also the strength of the magnets,I have been told that low levels will not harm your device, I have one on my case and so far no problem,I have had the case for a month now.