The iPhone 6 is currently in a final product validation phase before it begins mass production, Chinese media sources say. Apple is specifically said to be working with Foxconn's Zhengzhou facility in churning out small quantities, with the goal of making sure quality control is up to par. Some reports had indicated that the iPhone 6 was already in production, but it may be that rumors of delays were accurate.
Apple likely has a limited time to start mass production. The phone is expected to be announced in early September -- possibly the 9th -- and ship a short time later. Substantial lead time is needed to ensure there are enough units ready for first- and third-party outlets in every launch country, and even then Apple often runs out of units within days.