Users of these tags simply affix them to the back of their phone and tap the phone on a piece of hardware at the retailer to pay. The purchase is subtracted from a linked PayPal account. A text message is sent to the phone user with the purchase amount and any relevant balance information. A paper copy is generated only if you ask. It also spits out coupons and sales information via text, to entice shoppers to come back.

It's a great way, says Bling co-Chief Executive Wenceslao Casares, for retailers to tailor the shopping experience for you -- and learn more about your spending habits and preferences along the way.

Tia Gao, an MBA student at Stanford University, says her trips to the local coffee shop have gone up in the past three months since she's started using the Bling tag and getting coupons from the shop on her mobile phone.

"I'm going more frequently because it's easier," she says, and the rewards are more immediate than with other types of payment. "I can go in, even if I'm running and don't have my wallet."

She says that convenience has -- at least initially -- made her spend more than she wanted to with Bling, but not enough to cause her stress. In fact, she says that ultimately she will be better able to track her expenses via Bling's Facebook app and text messages.

First adopters

Last year, MasterCard launched its own system of payment stickers that can be tapped on its PayPass terminals at 270,000 retailers, including CVS, Home Depot, Best Buy and McDonald's. No signature is needed for transactions under $50.

Since last June, its largest issuer of PayPass tags, Citigroup, has issued almost 300,000 tags to its credit-card account holders.

"It's been a huge success," Natalie Marin, a Citi spokeswoman, says of the tags, which are not available for Citi's debit accounts.

Are these stickers just the first and fleeting step in the move to a mobile wallet?

The real future of mobile payment, some say, lies with phones that contain embedded NFC chips that can beam and receive information at a distance of 4 inches. Nokia and Samsung already include swipe-and-go NFC chips on a few phones. Nokia has said these chips will be in all of its smart phones starting next year.

Apple is expected to incorporate NFC into the iPhone 5, which is expected this summer. With credit-card or other banking information already on file for a large number of iTunes users, analysts say, Apple could control a large chunk of this mobile purchasing power.

Indeed, 47% of Apple iPhone owners say they're interested in mobile payments, more than twice the proportion of online adults overall.

What's wrong with convenience?

For those who carefully track their expenses, set limits and protect their phone, the transition to mobile payments shouldn't pose any problems, analysts say.

But there is always the chance of fraud. Many people leave their phone lying around more casually than their wallet. If someone steals the phone and it has an automatic login, a thief could do some serious spending.

MasterCard's Woo says that banks can easily track a lost mobile phone and stop short unauthorized charging by disabling the application inside the phone. Most large purchases, analysts say, require use of a personal identification number.

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Still, Gartner's Wallin says there's a lot we don't know about how secure these payment systems are -- for instance, how easy it is to intercept NFC signals.

However, the biggest risk is just its accessibility and ease of use.

Gail Cunningham, the spokeswoman for the National Foundation for Credit Counseling, says her group always advises people that more plastic equals more temptation, so leave the cards at home. But in this case, she says, there's nothing to leave behind.

"No one's going to pull their phone out of their pocket and put it in the dresser drawer. This could be a potentially big problem for undisciplined spenders."