"My expectation is that there's not going to be a lot of people out there looking for another subscription."
That is true, especially for those iPad customers who are iPhone/AT&T users. I already pay AT&T monthly charges for 3G services for my iPhone. I am not going to use my iPad and iPhone at the same time, so why should AT&T charge me for two different plans?
Last year, Apple sold nearly 25 million iPhones, mostly with AT&T plans since consumers did not have a choice (other than hacking the phone to use another carrier). A study at Stanford University students found that 10% of their iPhone-carrying students considered themselves fully addicted to the device and 32% were worried they would become addicted (give it up, college students -- everybody is doing it!).
So how many of these 25 million AT&T customers does Stephenson think will also buy an iPad? I am guessing at least 2.5 million "fully addicted" iPhone fans.
Just on principle, I refuse to pay AT&T twice each month for what is basically a single data plan. AT&T is acting like my friends at Delta Air Lines: big bullies picking on their existing customers who have nobody to protect them. There is little we puny consumers can do to ward off the abuse of the big bullies within airlines and telecommunications companies (let's include oil and financial services firms on the bad-boy list for good measure).
Instead of being bullies, AT&T should admit they understand that their existing customers do not want to be cheated and charged double. If AT&T does not want to make the second device's data plan free, they could at least make it cheaper (like Sirius/XM Radio does for my multiple satellite radios).
I don't foresee AT&T dropping its bully ways, so buy the iPad with only WiFi services and let Stephenson's prediction about nobody paying for his 3G network be self-fulfilling.