According to GSMArena, Apple’s Maps application which is to replace Google Maps will only be available to the iPhone 4S and up in full functionality.
Apple’s keynote speech, launching iOS6, discussed a feature list which will put any mobile OS dev into a frenzy, reportedly is only available to newer devices.
One of these features is Flyover, part of the new Maps app in iOS 6. Flyover lets you see an aerial view of the major metro areas around the world in glorious photorealistic 3D. As you can imagine, rendering all that detail would require quite a lot of horsepower and as such only the iPhone 4S, iPad 2 and the new iPad will support this feature.
The other feature is turn-by-turn navigation. Unlike with the Flyover, however, there is no apparent reason why this feature would not be available on the iPhone 4 and 3GS as third party apps have been doing it for quite a while now.
As you can see, Apple is intent on keeping its best features on its newest devices, whether for technical reasons or for something else. Still, compared to what people on other platforms are used to, iPhone users should be happy that they are getting updates at all, even after three years.