Well, the usual concern with computers and magnets is magnetic storage, and the N8x0 doesn't have any of that. As for other effects, changing magnetic fields do induce currents in conductors; a magnet on a leather case is not going to be strong enough to do anything bad, but with a more powerful magnet, there may be a possibility of it affecting the device. I'm speaking from a purely theoretical perspective here - I don't actually know much about the N8x0's hardware.