It's here. Your phone will soon replace your wallet:
You’ll be able to tap, pay and save using your phone and near field communication (NFC). We’re field testing Google Wallet now and plan to release it soon.
Google Wallet is a key part of our ongoing effort to improve shopping for both businesses and consumers. It’s aimed at making it easier for you to pay for and save on the goods you want, while giving merchants more ways to offer coupons and loyalty programs to customers, as well as bridging the gap between online and offline commerce.
Because Google Wallet is a mobile app, it will do more than a regular wallet ever could. You'll be able to store your credit cards, offers, loyalty cards and gift cards, but without the bulk. When you tap to pay, your phone will also automatically redeem offers and earn loyalty points for you. Someday, even things like boarding passes, tickets, ID and keys could be stored in Google Wallet.
It's not a huge leap technologically. I wouldn't be surprised to see this be a very commonplace thing in 4 years.