WWDC has revealed all sorts of interesting tidbits today, including Apple's plan for bringing official tethering support to the iPhone. While the technical aspect of it has been known for a while, it wasn't until recently that Apple revealed tethering would become a possibility on iPhone OS 3.0, and now AT&T themselves have confirmed the functionality will be available as an option for their customers in due course.
They don't make any mention on whether they will lock out older phones and only allow the newer iPhone 3G S model to use tethering, but we'll assume for now that any model running OS 3.0 will have this feature. Given Apple's relationship with AT&T, we can also probably assume the carrier will want to sell tethering plans - they have long tried to separate data usage directly on your phone with data usage on a tethered laptop. Hopefully the companies will make a firmer announcement soon to clarify how tethering will work on the iPhone.