According to Apple, tethering is build into iPhone 3.0 but it's up to individual wireless providers make the service available. Here in the US, there have much anticipation since the brief appearance of Netshare last year. Since then, tethering requires some jailbreaking and tinkering.
The news of tethering was not presented during the preview and it was only offered up by Apple execs during the Q and A session. So, what does it mean in the summer when the iPhone 3.0 is finally available?
I'm gonna go out on a limb here and say that when the new iPhones, service plans, and iPhone 3.0 are launched, tethering from ATT will be available as well. It may require a separate tethering plan. Tethering may also be available in some countries.
So you can bet that in June or July, Apple will make a big deal about it once the pricing and plans are finalized. Apple is not used to providing information half-baked so the answer they gave, essentially it's up to the providers, is the best one they can give at this time.
It would have been terrible to announce that tether will be available. There will be questions of when which will ultimately get out of control. As it is now, Apple rumors run rampant. Imagine availability of a tethering plan that goes out of whacked and leading to speculation about iPhone release dates.
So tethering is coming along with new rate plans and new iPhones. And it'll come this summer.