When the Kindle first launched, you could only pick one up from Amazon.com itself. In the last year or so, Amazon has smartly expanded into brick and mortar retail chains. Now the Kindle will be available in yet another place, AT&T Wireless stores. As you may or may not be aware, the global 3G radio in the Kindle uses AT&T's network in the US.
AT&T will sell the $189 3G model of the Kindle 3 only. The Wi-Fi model wouldn't really offer them the same bragging rights. That's probably what it’s about. People live their Kindles, and AT&T may want to remind them that they have a hand in the product. It would be a pleasant departure from complaints about iPhone signal issues. Amazon is benefitting by getting the Kindle in front of more and more eyes.
One of Barnes and Noble's apparent advantages in launching the Nook was the retail environment they control. But they only have so many stores. Amazon can expand to many retail chains without competing with themselves directly on other products.