An unlimited data plan would be the perfect match for iPhone tethering, wouldn't it? While it's perfect from the user's perspective, it's not AT&T perfect world. That's why, when they changed the data plans, they set terms for tethering.
IPhone tethering cannot be used with an unlimited iPhone data plan. Instead, tethering requires AT&T's DataPro plan, which includes 2GB a month for US$25/month, and another 1GB for $10.
All cellular data - whether on the phone or via tethering - combines to count against your monthly allotment. For users who don't tether, this should be OK, but with tethering, you may need to keep an eye on your monthly data use.