AT&T pulls another b0n3r
It's all about failure to plan - again - on at&t's part. And here's the proof - well, to me it's proof...
Back in 2004 I bought a Sierra AirCard 860 and Cingular's "3G" wireless service. In St. Louis, at the Cingular store in the Cingular HQ building. Cingular promised HSDPA service with the 860; however, at their headquarters all you could get was EDGE. Period. And that's ALL you could get unless you were in San Francisco, parts of Los Angeles, Washington, D.C. and a couple of other "major markets".
Until the iPhone 3G was released, that is. Within 2 weeks of the 3G release, suddenly there was HSDPA service in all these other locations that were "dead" days prior. Funny how that worked.
Now that the iPhone 3G has been out a while, these same HSDPA areas are showing major suckage as far as network performance. It appears that the iPhone is saturating the at&t 3G network. Oh, yes, I can get full 3G speeds - at 3am. Forget about getting it during business hours though.
So tethering, while available "standard" from every other carrier on just about any phone (even my Sprint Instinct (which I call the "vending machine phone" because every "fun" feature is available as an extra cost subscription) tethers instantly out of the box) is NOT an option on the one phone that is sucking up all the 3G bandwidth already. It's probably a backhaul issue: I'm sure that the software upgrade for 3G was easy, but to have enough backhaul to handle it requires a real investment.
So, it's back to why did they do this? First of all, Apple has at&t b0ned with the iPhone. Everybody wants one because it's cool, but at&t gets nothing from the device sale, and worse yet, it is stuck with infrastructure costs to support the iPhone 3G.
Now in Europe, where 3G is the rule rather than the exception in the GSM market, this isn't an issue. However, if Apple releases a phone with tethering enabled in Europe, it will instantly get back to America, and at&t is still screwed. So at&t has them delay the application as long as it can so it can backfill the backhaul (or whatever) to make it worth while.
Finally, remember, what the carrier wants you to do is BUY SERVICES, not applications, from THEM! So expect at&t to address this with a new "unlimited" plan offer at US$80-90/mo for iPhones that will allow tethering, hoping that the price will keep all but the few business customers that "need" tethering out of the market until at&t does something to improve the network.
Me, I'll stick to Sprint. Yeah, they suck too, but at least my data is consistent and coverage blows at&t out of the water in nearly every market. And even the "vending machine" phone works as stated out of the box - and it's useful for making voice calls, too!