This from iLounge.com:

AT&T has announced that the upgrade prices for iPhone 3G S will be $399 (16GB) or $499 (32GB) for customers who are still under contract after purchasing the prior-generation iPhone 3G, a $200 premium over the “standard” iPhone 3G S prices of $199 and $299. Using a pricing configurator found on Apple’s BuyiPhone.Apple.com web site, users can find out their individual upgrade prices based on the remaining length of their AT&T contracts, which may be higher or lower depending on the date of their prior upgrades. Users must commit to a new two-year contract; an $18 Activation fee and a $18 Early Upgrade fee are also assessed. More details to come.

In conjunction with Apple, AT&T today announced details of its rate plans for the iPhone 3G S, which will continue to offer “unlimited data” access but without any promises of support for tethering or multi-media messaging (MMS). The iPhone 3G S rate plan will cost $30 per month, with speed performance varying by area—an Apple press release notes that higher-speed 7.2Mb/s HSDPA will be offered “where available” without adding additional information. Tethering, a feature that enables the iPhone 3G S to work as a modem for a Mac or PC, is supported by the iPhone OS 3.0 but has been given no date of availability by AT&T, while MMS support will be coming in “late Summer,” with no clear details on pricing. AT&T is the exclusive carrier of the iPhone 3G S in the United States, as it has been for the original iPhone and iPhone 3G; users have complained about slow data speeds, dropped calls, activation computer issues, and other aspects of the AT&T experience during and after both prior launches.