While branded prepaid debit cards are nothing new, web-based game company Zynga has announced a new rewards program using American Express' Serve payment service that brings virtual currency to its games. What differentiates Zynga's payment system from services like PayPal is that Zynga will offer in-game perks and rewards for out-of-game spending activity. For example, starting this week, people using Zynga's card that spend $25 or more will receive "Farm Cash," a virtual currency they can use to purchase goods within Zynga's popular game, FarmVille. This is a nice co-branding opportunity for Zynga and American Express, but will FarmVille farmers change their personal spending habits to gain an in-game advantage?