I’m with DT, but if they can build enough, an 8G 3GS should sell very well. It makes it the safe choice at a low price for anyone getting their first smartphone.
Thinking from Apple’s logistics point of view, it’s all about model transitions with a long supply chain via carriers and their retailers, preventing Apple’s trademark overnight model replacement, and about managing final production allocation between sales and repair/warranty inventory. The 3G needs to be gone before the next transition, and the 3GS to have an entry level transition version, that does not directly devalue the price point at which 16/32G 3GS customers buy just before the transition.
Despite the difficulties of overnight model transition, Apple still has competitive advantage in iPhone product transition. Other handset makers have numerous models, each with physical customisation for country, and for carrier. They/their carriers are forced to discount to clear old inventory, slowing new product launches and driving new model price points down in sympathy. Apple has none of that; finished goods just needs firmware and packaging changes between countries. Stock exhaustion can be relatively easily synchronized worldwide.
Edit: of course it’s unclear if AT&T’s network can handle many more iPhones!