I stay in India where we don’t have AT&T and I still know that they are dumb. I recently visited US on a business trip and took a AT&T sim there. I used it for 10 days, without any issues, and came back to India. Once I came back I switched to Airtel. Some days later I tried switching on Internet connection sharing on my Windows Phone 7.5 phone, it is a HTC Radar, and guess what came up, I got an error message that to enable tethering I need to goto att.com/mywireless or call 611. Now, I was not even on AT&T network, so how dumb are AT&T to screw up my phone like this with no option to fix it.
To get my phone back to old state I had to reset my phone. I lost all apps and data. But I got all my contacts back, thanks to Windows live.
Now again, how dumb is AT&T. After reading on internet I understood that they charge $30-$45 to enable tethering, that is also so weird. I would somehow never go for a plan like that. Here in India, I am free to do what ever I want with my phone.