The surest sign yet of the impending iPhone 5 release is the reported drying up of iPhone 4 inventory, coupled with the fact that Apple's online store is currently down, triggering speculation that a new product launch is imminent.

The online store had a note Friday morning saying: We are busy updating the store for you and will be back shortly.

Multiple rumors say that production of existing models of iPhones has been stopped and that Apple's Asian manufacturers have ramped up iPhone 5 production.

From production rumors to the reported loss of an iPhone 5 prototype, and from the emergence of the first photo taken on the new device to leaks from retailer sources, there are loads of hints that say iPhone 5 will be released in early October.

Almost everything is in the realm of speculation as Apple has characteristically remained silent about its iPhone 5 release plans.

There have been numerous reports that pegged the iPhone 5 release in October. Reinforcing the October launch theory, CultofMac reported that Apple hold a press event to launch iPhone 5 on Sept. 21, which will take place in the San Francisco Bay Area. Press invites for the event could be issued on Sept. 14, the report says, adding that pre-orders will be taken Sept. 28.

Right now, here are the various pointers that strongly suggest iPhone 5 will see the light of the day in early October, or even earlier.

Apple Store Down

A BeatWeak report said the fact that Apple was updating the online store hinted at an “imminent” launch of a new hardware product, and suggested that iPhone 5 was one of the probabilities.

The fact that Apple online store is down and being updated doesn’t exactly confirm that Apple could launch iPhone 5 through the online store immediately. But such a scenario cannot be completely ignored, according to tech writers.

Bill Parmer, writing in BeatWeak, says that there is some chance that Apple could do so, considering that one European carrier began taking online preorders for the iPhone 5 this week. Though Apple chose not to launch the iPad 2 through the online store, the incident of an iPhone 5 prototype getting lost could change the situation drastically.

Apple may also be eager to launch the iPhone 5 this week after a prototype test unit went missing, he writes, reasoning that Apple would not want to be upstaged by third parties who might get hold of the device and show it off to the public.

Drying up iPhone 4 Inventory

Reports say it has become harder now to get hold of an iPhone 4 or iPhone 3GS, indicating that production of these devices has probably been stopped. iPhone 4 orders in the U.S now take one to three business days to ship, indicating that Apple is expecting a slowdown in iPhone 4 production as the next generation comes in.

Production Talk

Reports have said Apple's Chinese subcontractors Foxconn Technology and Pegatron Technology have started production of the iPhone 5. The latest is a DigiTimes report that Foxconn Electronics is producing up to 150,000 iPhone 5 handsets per day.

Japanese tech Web site Macotakara had reported earlier that Foxconn and Pegatron started production of the iPhone 5, adding that the assembled iPhone seems to be stocked without packaging. This is because Apple's new operating system, iOS 5, is not yet ready to install.

So what will be the time lag between this phase of production and the start of shipping?

According to various reports, Apple has begun iOS 5 and iCloud training with retail store employees. 9to5 Mac says Apple Stores usually begin their training on new software in the weeks leading up to a respective product launch. Apple beginning their iOS 5 training within the last few days could mean a launch sooner than the recently rumored dates of mid-to-late October.

And Computerworld has reported: Once iOS 5 is installed on those iPhones piled up in the warehouses beside the Pegatron and Foxconn factories, then we can expect the first units to hit retail a week or so later, suggesting Apple to be on schedule to achieve an early October launch of the phone, with the software released slightly in advance of that.

Pre-order Talk

Germany's Deutsche Telekom has reportedly started taking pre-orders for the next generation iPhone. Also, Best Buy employees have been informed that preorders for the iPhone 5 will begin this week in time for a release in the first week of October. Tech web site BGR reported that a Best Buy document showed that Sprint will be carrying the iPhone 5 at the launch of the new device.

According to rumors, iPhone 5 will have a teardrop design, will be slimmer and lighter, and have an edge-to-edge 4-inch retina display. It is supposed to have an aluminum plate casing, Apple A5 processor, 8-megapixel camera, HSPA+ technology for 4G support in AT&T, iOS 5 operating system with 200 plus features, world phone with SIM-less design and possible NFC support.