updated 04:16 pm EDT, Tue August 19, 2014
Three main candidates exist so far
A "plist" preference file in the past two Xcode 6 beta SDKs may point at possible resolutions for the iPhone 6. Specifically, the file is connected to the iOS 8 homescreen, and refers to a 16:9 resolution of 414x736. Because the iPhone SDK handles hardware resolutions through point values, actual Retina resolutions should be two to three times greater. That suggests that 828x1472 and 1242x2208 are possibilities, both of which would have enough pixel density on 4.7- and 5.5-inch screens to maintain Retina status.
Those two options join the 960x1704 resolution discovered earlier, also in the 16:9 format. The 1242x2208 is considered the least likely of the three, but isn't out of the realm of possibility. Rival phones like the LG G3 are already hitting resolutions up to 1440x2560, though the extra resolution hasn't been seen as a significant differentiator from the iPhone or other Android phones.