Know how some people are complaining that they have trouble connecting to AT&T's 3G network? How they drop calls? How they blame Apple? (Despite the phone working pretty dang well in other countries on other carriers). Remember the theory that there were so many iPhone 3Gs hitting the market that AT&T couldn't handle the load? (That their network was basically rabbit ears tied to old antennas? -- okay, we made that last one up!)
Now imagine that each and every one of those iPhones, especially in high-density areas like San Francisco, Los Angeles, and New York -- where each tower is already slicing bandwidth to razor thin margins -- suddenly found itself barraged by laptops tethering themselves on for the ride.
That might just be what's keeping AT&T from allowing iPhone tethering: fear it will crush their already strained 3G network. MacBlogz claims to have a source saying just that (via Gizmodo):
“Regardless of how many billions of dollars AT&T pours into their 3G network, it hasn’t been stable enough to handle all you iPhone users.”
