First, he promised that AT&T could handle any demand placed on its network by Apple's iPad, and now he's suggesting that it won't be that bad anyway. In a Reuters article, Stephenson suggests that the iPad will be a "Wi-Fi driven product," and that "there's not going to be a lot of people out there looking for another subscription."
That may in fact be true, and while I'm almost willing to offer him the benefit of the doubt that the majority of browsing on the iPad will be done via Wi-Fi, I also doubt that those who elect to go with 3G won't be using the heck out of it. Maybe AT&T thinks the extra charges from the non-subscription usage will shore up the network, but it certainly sounds to me like Stephenson isn't too worried about AT&T's 3G networks, and from both our experiences with them on the iPhone and the expected popularity of the iPad, he probably should be.
Then again, maybe he's got to say that it won't be a problem -- suggesting that AT&T might not be able to keep up with service demands probably isn't the best thing for the CEO of the company to do. But it sounds like AT&T is underestimating, publicly at least, the kind of 3G network traffic the iPad will bring.