As of around 30 days ago, the iPhone 3GS has been on the market for three years straight. This is the longest any of Apple’s iPhones has been for sale and makes it, I believe, the longest selling smartphone model still offered from any major manufacturer.
The iPhone 3GS has legs, for sure. I still have one kicking around and it’s surprisingly quick and useful. I can certainly see Apple releasing a new iPhone this fall, making the 4S the mid-range model, and hanging on to the 3GS on the low end. The iPhone 4 gets kicked to the curb, and no one notices.