Don't worry-- many cases use magnetic closures. The magnet isn't strong enough to cause data loss, and they're generally not located close to the brains of the PDA.
I don't think it could really affect your device anyway - while things like speakers, CRT monitors and hard-drives rely on magnets in some form or other, sold-state memory and LCDs don't need them. While a strong enough magnet may well cause data loss of some kind (due to the circuitry), I sincerely doubt a magnet that strong would be needed to hold a case latch closed - you'd probably never be able to open it!