I guess it's good if they're giving customers more "choices", but I think from the buyers' point of view, buying a 3GS, no matter how more "affordable" it is, is a very misguided choice.
If there was such thing as a "wifi" model an iPhone, then perhaps, maybe yes, that might be a good idea, but as far as I know, even the 3GS requires monthly data plans. AT&T offers the cheapest one, which is $15 / month. If you use your phone for a year, that is already enough to completely dwarf the money you paid for the phone, so I'd really advise people to just make a good investment for a new phone and get a 2-year contract.
That's the thing about smart phones. It's not like in the pre-smart phone era where the price of the phone actually determined whether or not you're going to be able to afford one. Nowadays, when you opt to purchase one, you're basically promising yearly spendings of over $100 (or much more) just for using your phone (and of course, that plus the actual phone minute plans and whatnot).
So, sorry to digress, but I just don't see the merit in buying a 3GS, no matter how affordable it is. If you can't afford $200 for a phone, then you should probably really look into how much the data plans will cost you in the long run.
(I know some people want iPhones very badly on non-iPhone companies like T-Mobile, but honestly, I'd say no matter how good the iOS is, it's better to either switch carriers that has an iPhone 4 or just get an Android phone than to buy a 3GS).