The only way that a magnet could erase data that I know of is if it is on a media that is magnetic in nature. Of course I suppose that a big enough magnet could do something but I would think that it would have to be HUGE! In general the magnet thing was based on an issue with PC's due to the magnet disks they use.
the case I got is the original case from hTC the two magnet is so powerful that I can stick the case & the phone on my frig. I have been using it for three days now since I got the case and the micro sd & the Tilt is still working. hope it's ok
I would feel pretty confidant that if it is the case that came from htc then the magnet is ok to use. pretty useful actually to stick it to the fridge. should be easier to keep track of that way.