Turn out the lights, the party's over - at least, it is for Apple's third-generation iPhone, the 3GS.  According to supply chain sources who spoke to IT Pro, Apple has decided to discontinue production of the 8GB iPhone 3GS. A source also sent 9To5Mac a screenshot of an internal document stating "the iPhone 3GS is currently unavailable, we'll keep you updated."

The 3GS was released in June 2009, and proved a huge hit selling over a million units in the opening weekend. Despite becoming an entry-level device after the launch of the iPhone 4, the 3GS has continued to sell very well, with estimates suggesting Apple was still shipping over a million units a quarter.

It's not too surprising the 3GS is being retired, though, as it is over three years old. However, its core specifications, which include a 600MHz processor and 256MB of RAM, are not too far off the latest iPhone 4S (1GHz/512MB). This has allowed the third generation device to receive all the major over-the-air updates for iOS, making it one of the best supported handheld Apple devices to date.

With the iPhone 5 set to launch with iOS 6 next week, there were suggestions the 3GS may be provided with an upgrade over-the-air. However, sources confirmed to IT Pro that Apple has decided to retire the device to focus on the production of the iPhone 4, 4S and 5.

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"The 3GS does not have core hardware features such as a front-facing camera, so it will not be able to allow users to make video calls over 3G - a feature Apple is pushing with iOS 6," the source told IT Pro. "Once retailers and operators have burnt through their stock of the 3GS, they will not be replenished by Apple."

Retailers and operators could slash the prices of devices to clear inventory and the price of the iPhone 4 is also likely to drop once it becomes the de-facto entry-level device.

Some reports have said that Apple will retain the 3GS as its cost-effective pre-paid option. Others have reported that Apple is preparing two next-generation iPhones for this fall: one as the successor to the iPhone 4 and one as a cheaper, possibly pre-paid model.

According to several sources in different countries, stock of Apple's iPhone 3GS is quickly tightening with hardly - or even any - shipments coming through to certain regions. Also, according to 9 to 5 Mac who spoke to a source at one popular international carrier, the iPhone 3GS has disappeared from the stock database.

The Telegraph reports that Apple is likely to drop the iPhone 3GS from its lineup once the company introduces the iPhone 5 next week, leaving room for the iPhone 4 to slide down to the free-on-contract price point currently occupied by the iPhone 3GS. The report also indicates that Apple may introduce an 8 GB version of the iPhone 4S to occupy a middle price point as the company did last year with the iPhone 4.

It would certainly not be a surprise for Apple to discontinue the iPhone 3GS in order to keep its iPhone family at three separate devices with the cheapest being offered for free on contract. Shifting the iPhone 4 into the position currently held by the iPhone 3GS would also give more carriers the ability to offer a free iPhone, as the iPhone 4 is available in both GSM and CDMA versions while the iPhone 3GS is only available as a GSM device.

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The iPhone 3GS would likely be most missed by contract-free customers, prevalent in high numbers in many emerging markets. Apple currently sells the unlocked, contract-free iPhone 3GS for $375 in the United States and at somewhat higher prices elsewhere such as $455 in China, and it could presumably drop that price even further if it elected to continue offering the device alongside the three other iPhone models.

Apple has declined to comment on their production cycle, stating that it does not respond to rumors and speculation.