My math:
More iphones==>more MMS==>more SMS==>more tethering==>more monthly charges.$$$
Do you try to make your money now by charging more for the phone and attracting fewer buyers / subscribers or charge a lower price and rake in monthly fees over time?
I don't know what makes more fiscal sense, but the American way is to pLay now and pay later.
Your thought process is in error. New customers and those that are upgrade eligible get the same price you paid for the 3G last year. Why would they charge less on the 3GS so you can upgrade from your 3G to pay the same monthly fee you will already be paying for the next 12-18 months.
How do they make more money by letting you get a 3GS? Really, come up with a better argument, your idea has more holes in it than a stripper's stockings.