updated 10:38 am EDT, Mon April 14, 2014
Existence of molds suggests Foxconn almost ready to go into production
(Updated with Nowhereelse mold photo) Photos posted to Chinese site Sina Weibo show what appear to be schematics and case molds for a next-generation iPhone. The schematics indicate a design roughly consistent with current iPhones, including touches like chamfered edges and thin bezels. It's difficult to determine size, but one obvious difference is the phone's depth, which is even thinner than the iPhone 5s.
Perhaps even more significant -- if the leaks are authentic -- are the images of the case molds in both finished and CAD-rendered form. Their existence suggests that Foxconn (the reported source of the leaks) has received final or near-final specifications, and is almost ready to go into production.
Assembly will likely only begin in June or July at the earliest though, since Apple isn't expected to launch any new iPhones until September. The company is even rumored to be doing a staggered release of its 4.7- and 5.5-inch models, the latter of which might not arrive until November, owing to production obstacles.
Update: French site Nowhereelse.fr has obtained a photo of a mold next to an iPhone 4S for comparison. It's believed that mold is designed for a 4.7-inch iPhone.