Just for your interest: BlackBerry is using magnetic sensors in their devices since years, they are also producing holsters and pouches with built-in magnets. These magnets toggle the sensor to either wake the phone of put it to sleep. I don't know a single case where a BB suffered any damage related to those magnets, neither the device itself, nor the used memory card. Since they don't really use different components than any other manufacturer out there, I wouldn't worry about those magnets.
Keep in mind even some Windows Phone flipcases come with magnets, I just ordered a Nillkin flipshell which uses a magnet to keep the cover closed.
Yet I'd be careful with any heavy-duty magnets of course!