Visa and MasterCard are working on advertising strategies that would target people based on their buying history. When it comes to privacy, the companies may be treading on thin ice. The Wall Street Journal describes a scenario where you might swipe your card at a fast food restaurant, which then triggers an ad for weight loss to show up on your computer. If either company can pull it off while keeping our private information private, there are mountains of data worth even bigger mountains of advertising dollars to be had. And if you're troubled by this, well I guess it's time to start paying in cash.