According to reports, Apple's main manufacturer, Foxconn, is currently pumping out a whopping 150,000 iPhone 5s a day. That's a lot of iPhones!
That points to the release of the next generation iPhone being relatively soon, either later this month or early next month. Further lending credence to this assumption is the fact that, earlier today, Apple's shipping time for the iPhone 4 went up to three days from immediately. It's back to immediately now, but Apple may be ramping up to switch over to a whole new model. They should be announcing a press conference any day now at which all details will be revealed.