PayPal has launched a mobile payment service for e-commerce merchants, and retail giants Buy.com and Nike are among the first to utilize the new system. Mobile Express Checkout is designed to provide a seamless payment experience for consumers shopping from their iPhones and Android devices, and PayPal says the service will be available to all online retailers by the fall.

In a company blog announcing the launch, PayPal indicated that its mobile transactions have risen from $25 million in 2008 to $141 million in 2009, and it projects around $500 million in mobile transactions this year. As an industry, experts have predicted the mobile payment business to exceed $633 billion by 2014.

Shoppers need only enter their PayPal account logins when using Mobile Express Checkout, expediting the payment process by eliminating the need for reentering shipping or billing information.