These cards carry a code that when entered in iTunes, this recognizes the amount of the card in order to purchase content from the iTunes Store. Well, it turns out that in China they have managed to hack the algorithm of these cards, being able to sell them at a low price but that when putting the code in iTunes a much higher balance appears. For example, the card costs you 17 dollars but you will have 200 credit.
I suppose Apple will soon do something because if this spreads it can be very dangerous for the viability of the iTunes Store.
VÃa: applesphere