Preview at WWDC likely in June, followed by September launch.
iPhone 6s Now Available SIM-Free in Apple Online Store
The SIM-free iPhone comes without a wireless contract commitment or carrier financing. It doesn’t come with a nano-SIM card for iPhone 6s, iPhone 6s Plus, iPhone 6, iPhone 6 Plus, or iPhone 5s. And because it’s unlocked, it isn’t tied to one carrier, so you can get a nano-SIM from any supported carrier worldwide, such as AT&T, T-Mobile, Verizon, or Sprint in the United States.Purchasing a SIM-free iPhone means that customers have to pay for the full price of the device upfront. It's ideal for customers who want to avoid multi-year commitments with carriers or for travelers who want to easily be able to switch to local carriers when traveling abroad. The SIM-free iPhone 6s is model A1633 while the iPhone 6s Plus is model A1634.