According to the New York Times, American Express and Riot will be officially announcing the League of Legends prepaid debit cards along with some sort of formal sponsorship later this week on Wednesday, August 14, 2013.
Debit card users will gain 1,000 Riot points after signing up for the card and will receive an additional 1,000 points for adding $20 to the card. Ten thousand points will be given for linking the cards to a direct deposit account, and more bonus points will be given out for the first ten purchases made.
After that, the card will dole out rewards for every dollar spent and will otherwise function like a regular prepaid debit card.
This whole affair sounds ideal for a serious League of Legends player who spends their loose change on the game.