He told me that the original offer posted at DeafMac never existed and was, as posted, a mistake. AT&T does not offer an iPhone-specific data-only plan at this time: not for hard of hearing, not for the general population. They are working to have one in the near future.
The rumor about hearing customers overwhelming the plan and destroying opportunities for the hard of hearing community was just that, a rumor. No such incident occurred. May went on to explain that AT&T offers a variety of text accessibility plans (TAPs). These plans are available for a number of devices including Blackberries and such but not the iPhone. May was unable to tell me what it takes to qualify for such plans, whether a doctor's note for example was required. He promised to get back to me with an answer about qualifications and I will update this post when he does.
AT&T offers a hard-of-hearing hotline for customers who want to learn more about accessibility options. You can call them at 866-241-6568 (Voice) or 866-241-6567 (TTY).
Thanks to Michael Coe and Warner May.
Update: I'm told that iPhone TAP plans, when and if they exist, will only be offered to deaf customers. Customers will be sent certification paperwork, which they fill out and return to the call center. Once received and approved, either a plan rate change is applied to their current plan or the call center can help the customer set up a new activation. Paperwork (when this goes live) can be submitted via TTY, email, fax and postal mail.