There was a brief period during the waning days of summer 2008 when you could buy a tethering app for the iPhone 3G. Tethering is technique that lets you share a phone's wireless data service with another device such as a notebook. However, Apple pulled the app from the iTunes App Store within a few weeks (presumably at AT&T's request). If you want to do that today, you need to jailbreak your iPhone and purchase (or otherwise obtain) a tethering app outside of the conventional iTunes store route. Apple's iPhone OS 3.0, due out tomorrow (Wednesday), provides tethering as a supported feature. However, AT&T does not provide a mechanism to let U.S. iPhone owners to use that feature.

A number of people (including me) assumed that Sprint would provide tethering a feature with the recently released Palm Pre. However, this is not the case. And, it looks like Sprint doesn't even want people to discuss the development of such a feature according to this item in preThinking...

Sprint Warns Against Palm Pre Tethering Hacking

This is a shame since Sprint has a large 3G footprint in the U.S. with, I'm told, reasonably fast 3G wireless data throughput.