The iPhone 3GS is about to come out of stock. After that, it will never come back. Apple has confirmed that the 3GS model will be discontinued in the coming weeks. The stock will then be filled only with iPhone 4, iPhone 4S, and iPhone 5.
The iPhone 5 will be released September, 21 in U.K. Then the 8 GB iPhone 4 will be the oldest iPhone model in the lineup.
This decision may come also due to the release of iOS 6. Even though you can install the new mobile OS on an iPhone 3GS you will not be able to use all the features because the old components of the smartphone cannot cope with its need of resources.
Anyway, technology moves on but old smartphones get stuck in the mud. The 3GS model is 3 years old and outrun. Its demise was predictable.