"Digital wallets" that let consumers pay with the swipe of a smartphone could make the plastic credit card obsolete. But the technology also could chip away at consumers' privacy—and tempt them to spend more than they otherwise would.

Using a technology known as "near-field communications," or NFC, consumers will be able to buy items simply by passing their phones in front of a sensor at the checkout counter. Though NFC isn't available in many phones yet, a number of companies, including Google Inc., are close to rolling out programs.

By 2014, 70% of U.S. consumers will have smartphones and 52% of those will be NFC-capable, according to estimates by payment consultancy Mercator Advisory Group. Already, app developers, wireless carriers, banks and card companies are racing to shape the potentially lucrative mobile-payments ecosystem.

Here is how it could play out: A man on his way home from work gets a notification on his smartphone, reminding him that it is his wife's birthday. The phone knows he hasn't bought her a gift yet. It sends him a 15%-off coupon for roses at a nearby florist where he has shopped a few times in the past year. The coupon lures him back to the store, where he buys a bouquet, perhaps spending more than he originally intended.

The problem, some critics say, is that the ability of merchants, coupon services and others to extract more information about where people shop and what they purchase amounts to an invasion of privacy.