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3Gs will have to support iOS 6 if they keep selling it or make a lot of uninformed customers mad then again they may never know iOS 6 came out.
This is a real issue. If apple wants to keep it's reputation as supporting it's customers (for a reasonable phone lifetime- Essentially the life of contract), it will need to support at least one add'l major iteration of software beyond purchase date.
In other words, if you can still buy a 3GS in 6 months when iOS 5.X.Y is out, I highly doubt that apple wouldn't support iOS6 on the handset. With a totally different chip architecture, not even used on new ipod touches these days (Is there still a cheapo without and A4?), I think this is unlikely. I also highly doubt that apple wants to continue to support lower 640 X 480 resolutions into iOS6.
My guess? The cheapo phone might look like a 3GS (or might just be a slightly crippled iPhone4). I would guess it will have a spec bump to an A4 chip, more memory, and a retina display. I would also guess no FF-Camera and a mediocre rear facing camera.
As component costs come down, this is realistic to do. Especially if it expands to a whole new world of consumers previously untouched- The ones firmly expecting a phone for free w/ contract. I'd be shocked if they couldn't eek out 40+% margins on these things...