As reported earlier this week, Virgin Mobile will begin selling the iPhone as a prepaid device — and the Sprint division has just announced that it'll happen on Friday, June 29th (a week later than Cricket, which kicks off on June 22nd). The 8GB iPhone 4 will be available for $549 and the 16GB 4S for $649, contract-free, with no activation fees. Three plans will be available at $35, $45, and $55, ranging from 300 to unlimited voice minutes, unlimited messaging, and "fair use" data of 2.5GB per month (you can save $5 on each plan by signing up for automatic payments). You'll also be able to tack on tethering for an additional $15 per month.