I don't know why more people haven't thought this through, but:
1st generation: the original iPhone
2nd generation: iPhone 3G
3rd generation: iPhone 3GS
4th generation: iPhone 4
5th generation: iPhone 4S
6th generation: ...
The next iPhone model might be the iPhone 6, or the iPhone LTE, or the iPhone WiMAX, or some rebranding we can't foresee. But there is no way it's going to be called the iPhone 5. That's not the way Apple thinks.