The iPhone 6 and iPhone 6 Plus were just announced. But as usual, phone specs never give the meta viewport
(css) device-width of iPhone 6 is 375px, of iPhone 6 Plus is 414px. Note that iPhone 6 Plus report
window.devicePixelRatio = 3 (while not truly at 3 DPR)
Update 1: just to clarify, I believe this is NOT a prediction, but actually tested. See more detailed and reputable reference. If in doubt, you can wait for device to be out.
Update 2: for users with Display Zoom enabled on iPhone 6, css viewport in Mobile Safari fallback to 320px (like iPhone 5); for iPhone 6 Plus, it fallback to 375px (like iPhone 6).