It seems to me like the 'S' iPhones are beta iPhones for the next major iPhone. The 3GS was beta for the 4 and the 4S is beta for the 5.
With the S iPhones, they incorporate some new features and try them out on the public, improve on them and add them to the next major release. Siri is being release on the 4S, but it's still in beta (according to the keynote). Of course, with the iPhone 5, it will have several more languages and hopefully a translator too. Anyway, I still think the iPhone 4S is a beta iPhone for the iPhone 5.
4S is still a great phone though.
With the S iPhones, they incorporate some new features and try them out on the public, improve on them and add them to the next major release. Siri is being release on the 4S, but it's still in beta (according to the keynote). Of course, with the iPhone 5, it will have several more languages and hopefully a translator too. Anyway, I still think the iPhone 4S is a beta iPhone for the iPhone 5.
4S is still a great phone though.