Virgin Mobile is dipping its toe in the brand-new prepaid iPhone space, taking a huge gamble on the device by putting up an unlimited (well, mostly) data plan that starts at $30.

The company, a subsidiary of Sprint, will offer the 16GB iPhone 4S for $649 and the 8GB iPhone 4 for $549. The plans come with talk and text, as well as unlimited data that may be throttled past 2.5 GB per month. The $30 plan includes 300 minutes of voice. A $40 plan offers 1,200 minutes, and the $50 plan adds unlimited talk to the deal.

Virgin joins Cricket in offering U.S. customers a month-to-month iPhone plan rather than a contract. Cricket’s data plan for the phone is $55; its prices for the iPhone 4S and iPhone 4 are $499 and $399, respectively.

Virgin will begin selling the iPhone on June 29.

By offering the iPhone on prepaid plans, Apple is able to tap into a demographic that may have been putting off a smartphone purchase because of the two-year commitment required by postpaid carriers.

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