There's one main reason I'm not going to buy the iPhone 3GS.
It's not that it's not enough of an upgrade. It's a decent revision that makes the phone a heck of a lot faster. Sure most of the obvious parts of the upgrade is in the OS revision, but there are still a couple great features of the iPhone 3GS. The new camera is fantastic. The compass is great and should add a ton of functionality to maps and game controls. The extra memory, faster processor, and extra battery life is killer!
But I'm still not going to upgrade to the 3GS, even though I really want to.
The reason I'm not going to upgrade is because of AT&T.
AT&T has consistently treated iPhone users as second class citizens on their network. Not allowing iPhone applications to do things that their counterparts on Windows Mobile or Blackberry do. And the whole time, charging us more for the privilege of having less service.
They have announced that they will be adding MMS support for the iPhone in late summer. Nice of them to say that it will be at no extra charge. Tethering support will come and some unspecified time later and will be an extra charge. My guess for the price of tethering is just take the cost for Windows Mobile tethering and add $15.