I think the iPhone was a little different story. There are a number of people around the world who have bought into the entire iPhone franchise. After a couple of years their phones go obsolete and they either have to leave their ecosystem for a new platform, or stick with iOS and get the latest and greatest that Apple offers.
The impressive sales figure say less about the market agreement of the validity of the 4s as much as it says that people are happy in their ecosystem and are ready for another iPhone and another 2 year contract.
The iPad is a different story. There are no subsidized rates, and no contracts with the exception of the wireless service on certain models. To look at the sales figures of the 4s and suggest a similar sales cycle is kinda foolish. We'll see in six months where the new iPad rates.