That's my point - this way with the 3GS as the entry level model it's clearly a cheaper phone, leaving the iPhone 4 users happy that they've still got the cache that goes with their more expensive version.
The best case for Apple is indeed great: The old phones will be clearly marked as less desirable, which allows Apple to sell to more price-sensitive customers, while also communicating to the faithful that they really need to ditch the old junk and buy new phones. Perhaps most importantly, they grow the market share of iOS.
But the potential downside is that there will be Apple-branded hardware out there that is seen as cheap and less desirable, and that many people won't understand the distinction. If that happens, the 4/4S guy will now just have an "iPhone" like the rest of the riff-raff, which opens up the high end to competitors.
Maybe, but it's a gamble. It's not enough that the 4/4S guy himself knows that he has the good stuff. Everybody else needs to know it too.