At their I/O conference last week, Google launched a dedicated website that outlines how customers at the checkout will soon be able to keep their wallets and mobile phones in their pockets and pay "hands free". Square (Wallet) had a similar service, but shuttered it a while ago. You can watch the Google commercial here.
Technical details have not been disclosed but the similar Square Wallet service used geofencing to connect customers to their phones for identification. The Square product was dropped last year after failing to win over either retailers or customers.