In what can be interpreted as an attempt to stanch a possibly hemorrhagic flow of customers flocking to the Verizon iPhone, AT&T has added an extra 2 GB per month to its phone tethering plan for the same US$20/month rate as the previous plan, which came with no extra data at all.
Note that AT&T's press release (and Engadget's coverage) doesn't specifically mention the iPhone getting access to this increased tethering cap, and the HTC Inspire 4G is getting first crack at the new plan starting February 13. However, if AT&T doesn't eventually give its iPhone customers the extra capacity in addition to its other smartphone customers, the company's frankly just shooting itself in the other foot -- its first foot's already been shot completely off by AT&T's notorious history with iPhone tethering.
It still baffles me that US customers have to pay extra for tethering at all; data is data, regardless of how it's pulled down from the network. Here in New Zealand we're not exactly known for having generous cost per GB plans, but at least our carriers' attitudes toward tethering are more pragmatic; there's no extra charge for tethering your iPhone, and the carriers have all said, "If you're willing to pay for the data, then tether away to your heart's content."
At least AT&T is finally adding some actual value to the tethering plan by including extra data, but it's still charging an awful lot of money for a service that other countries' carriers include for no charge.