I certainly wouldn't be surprised if that was the case that they're just going after high bandwidth users and not doing packet inspection (yet). I occasionally swap my iPhone 4 SIM to a Nexus One and use the built-in wifi tethering feature, and haven't heard from AT&T about it. On the other hand I only have used maybe 2GB of tethered data over the last year and in general run up no more than 800MB-1GB of mobile data a month.
There are people on that thread showing they used 10+GB/month, connecting their Xbox 360s to Live via their phones, etc. That certainly seems like a way to get "noticed" by AT&T.
edit: one guy on there pasted his usage from AT&T's account manager - 165 GB!
Those are the people who are ruining it for everyone.
I use the Tethering on my Nexus One (Tmobile) from time to time, but I don't abuse it. I use it as an emergency backup internet access for cases like if the wifi at the hotel isn't working, etc. IMHO, that's reasonable and my usage when tethered probably isn't much more than when I run things like Pandora or Youtube on my phone.
If I were using 10+G/month, then I'd expect to have to pay for a higher priced "tethering included" plan.
Some put it the other way around: you owe the ever-improving network to the heavyweight users [1]. Well, at least Cisco says that -- and you know what they sell.
In any case, it's not the other user that oversold the bandwidth; it's the ISP. They made an unhedged bet, it fell short of working out(predictably), so it's their turn to foot the costs of upgrading the network.
Recently I was traveling a lot, and therefore making regular use of TMobile tethering (I also have an N1). At one hotel in particular where I had a long term stay the wifi was very slow and intermittent, and so I eventually hit the point - 5GB I think - where TMobile starts throttling bandwidth (supposedly to EDGE speeds, but it felt slower than that).
I recognize that's an unusual amount of usage for one month and I would have gladly paid extra money to TMobile to continue getting full speed data, but they don't even offer the option to do so - I assume because that would require them to admit that the service advertised as 'unlimited 3G' isn't really.
There are people on that thread showing they used 10+GB/month, connecting their Xbox 360s to Live via their phones, etc. That certainly seems like a way to get "noticed" by AT&T.
edit: one guy on there pasted his usage from AT&T's account manager - 165 GB!