The iPhone 4S hasn't even launched yet, but it's already the fastest-selling Apple product in history, at least if you go by the preorders. That doesn't mean customers have abandoned Apple's older phones, though. Analyst T. Michael Walkley of Canaccord Genuity did some digging, and after checking around the US carriers he found that sales of the iPhone 4 and iPhone 3GS haven't slowed down even in the face of the new iPhone 4S's debut (hat tip to AppleInsider for their coverage of his analysis). In years past Apple has seen sales slumps in the quarter preceding the launch of new iPhone models, but even though the iPhone 4S launched later in the year than its predecessors, iPhone sales remained robust.
Walkley and other analysts have repeatedly found that old iPhones continually outsell new Android phones. Though it's well over two years old now, the iPhone 3GS continues to be second only to the iPhone 4 in US sales. Even though it's no longer a phone I'd recommend to new users, the 3GS is likely to remain popular given that it's now available for free with a two-year contract.