The world’s most popular smartphone is now available as a prepaid handset on Virgin Mobile. The 8GB iPhone 4 is being offered for $549, while the newer 16GB Phone 4S costs $649 with no contract. Both handsets are eligible for Virgin Mobile’s unlimited data and messaging plans that start as low as $35 per month. Apple’s smartphone is available through Virgin Mobile’s website, RadioShack, Best Buy and select local retailers. Regional carrier Cricket, owned by Leap Wireless, was the first carrier to offer a prepaid iPhone in the United States. The company is charging users $399.99 for the 8GB iPhone 4 and $499.99 for the 16GB iPhone 4S, besting Virgin Mobile’s hardware pricing by $150.