Google has just announced that Google Checkout will be shut down in the next six months. By 20 November 2013, it will be completely replaced by Google Wallet. According to Google, merchants who are currently dependent on Google Checkout will have to look for other payment processing services. Google partnered up with services like Braintree, Shopify and Freshbooks to offer discounted migration options.
Merchants (US only) who already have payment processing services are also encouraged to to apply for Google Wallet Instant Buy, which (according to Google) offers a faster buying experience for users who shop with Google Wallet. That's due to Google Wallet's ability to work across multiple platforms, from desktops to mobile devices.
Source: Google Commerce Blog