Ok, so let me get this straight. If you pay for unlimited data transfer through AT&T with an iPhone plan, right? And both Verizon and Sprint permit tethering. So if you pay for unlimited data, why is it not permitted? If a customer want something, be it a good or a service, it is in a company's best interest to provide said good or service to them, even if there is an extra charge for it. All AT&T is doing is just slowing down progress, while continuing on with all of this contract nonsense, IMO.
"Unlimited" in the plan's terms of service refers to data sent to and from, and used on, the device itself. This isn't new with the iPhone; all of AT&T's "PDA' data plans work this way. Tethering plans are available and cost more. AT&T does it to try and preserve some control over the network; the amount of data you'll send and receive from a PDA (iPhone) is in theory less than the bulk you'd do with a PC. PC's have MANY more applications that deal with much larger data.
Same idea as behind the App Store's 10MB limit on apps which can be downloaded via EDGE or 3G; anything larger must be downloaded via WiFi or synced from iTunes.
Think of it this way: Auto registration fees provide unlimited use of the road. However, states recognize that certain types of cars place a greater burden on the system, like semi trucks, and so the fees for those are higher. (I realize it's a simplistic analogy.)