updated 11:46 am EDT, Wed May 14, 2014
Jump needed to keep iPhone at Retina levels, but below some other phones
Apple is testing out a 1704x960 resolution for the iPhone 6, sources claim. The iPhone 5s uses a 1136x640 resolution for its 4-inch display; because Apple is believed to be working on 4.7- and 5.5-inch versions of the next-generation iPhone, though, Apple is likely being forced to up resolution to keep Retina-quality graphics. Apple defines "Retina" levels as anything over 300ppi; on a 4.7-inch iPhone, 1704x960 would yield 416ppi. On a 5.5-inch model the same resolution would still reach 356ppi.
If 1704x960 is adopted, that also means that Apple will be sticking with a 16:9 aspect ratio. Whatever ratio or resolution Apple picks, app developers will likely have to redesign their interfaces, much as they did for the debuts of the iPhone 4 and 5.
1704x960 is actually relatively low among high-end smartphones. Most devices at or over the 5-inch mark -- like the HTC One, Google Nexus 5, and Samsung Galaxy S5 -- use 1920x1080 screens. It's anticipated that some phones in the next generation will even go beyond that resolution, despite it being virtually impossible for the human eye to notice the difference.