If a device uses a traditional spinning disk then magnets can kill the disk. The iPod classic is an example of a device that if you take two magnets and put them on either side while it is in operation, you can pretty much bet that it is toast.
The solid state memory in other devices (iPad, iPhone, iPod Touch/nano) is not subject to magnetic damage. The bits in solid state memory are manipulated with electricity, not magnetism, so running a magnet across them has no negative effects.
The only negative impact an unshielded might have on an iPad would be a possible reduction of signal (BT, wifi, cell data), but I'm not sure even that is a concern...
Basically, if you want to get a big fat magnet and stick your iPad to the fridge - Rock on. You have nothing to worry about with the case