You're talking as if Apple continues production on the 3GS. Do they really, or do they access old stock? I'm genuinely asking as I don't know - I had assumed they accessed old stock.
See also Anandtech's writeup of the substantial new chip fabrication improvements going into this year's version of last year's iPad: http://www.anandtech.com/show/5789/the-ipad-24-review-32nm-a...