Naturally, both of these items are decorated with Pokemans, so friends who come anywhere near your DS-wielding hand or your designated DS Accessories Storage Area will be tipped off about your pathological need -- your addiction -- to catch 'em.
Also, it wouldn't be Pokémon if there were no incentive to buy both games -- in the case of these bonuses, the styli each have a different little Poké-mascot on top: Dialga for the Diamond version and Palkia for the Pearl version. We assure you that those are real Pokémon names and not rare diseases. Or maybe both? We don't have medical degrees.