For the three months ended June 30, prepaid card provider Green Dot (GDOT) disclosed in early August that Walmart (WMT) represented 62% of its total operating revenue, compared to 60% for the same three months in 2011. Green Dot cautioned in the filing, "A credit concentration may exist if customers are involved in similar industries, economic sectors, and geographic regions. Our retail distributors operate in similar economic sectors but diverse domestic geographic regions. The loss of a significant retail distributor could have a material adverse effect upon our card sales, profitability, and revenue growth." Green Dot shares are down 22% in pre-market trading after Walmart announced a prepaid card agreement with American Express (AXP). Walmart, however, said on today's conference call that its program will Green Dot will continue to grow. NetSpend (NTSP), another player in the prepaid card space, is down 12% to $9.50 after Walmart and AmEx announced their card deal.