Google will unveil on Thursday a mobile-payment system that allows consumers to pay at checkout using their phones, rather than debit or credit cards. Initially, the feature will only be available on Google’s Nexus S phone. It will receive a Citibank-issued MasterCard credit card number and a virtual Google MasterCard prepaid card. It can be used at any of the 124,000 with a MasterCard PayPass terminal. Some retailers, like Subway, American Eagle, and Macy’s, will upgrade their terminals to participate in rewards programs.