I received the email and text from AT&T and I'm still tethering. Here's why...
We can discuss the legality or morality of tethering or not tethering all day long. You signed a two year contract for the service and for most of us on the unlimited plan, the contract is up unless you purchased a new phone and AT&T made you up your commitment. The contract is in place so you don't bail on them after getting a great "deal" on a new phone. The contract you sign is different than the TOS. The TOS is an ever changing document created by AT&T. Did anyone look at the TOS when they first got unlimited? I did and there's no mention of tethering. Why? Most phones couldn't tether, including the first generation iPhone. Violating the TOS isn't violating a contract. What it might lead to is AT&T changing the terms of your service. They can do that at anytime. AT&T could send a letter to all customers on unlimited and say effective next month, there is no unlimited plan and everyone will be converted to a limited plan. They are well within their rights; however, they are not doing that. With the currect email/text, AT&T is banking on people switching to a limited plan or reducing their data usage. AT&T didn't think through the unlimited plan when offered or they didn't realize the popularity of the iPhone, Pandora, Netlix, etc. These services just didn't exist for phones. This is AT&T's problem.
I contacted two representatives with AT&T when I received the email. They said they thought I was tethering because of my high data usage and a spike in my usage. True. I was using about 1-2 GB and then went to 5 in February 2011. That's because I started using Pandora and Netflix more on my iPhone.
My wife got an email and doesn't tether. She uses less than 500mb per month on her unlimited plan. My guess is they are targeting high users and those on unlimited plans.
AT&T can check to see where the data is coming from, but my guess is they aren't since that would involve "packet sniffing" and be an added cost to them. That would violate your privacy as they would be able to see what addresses you are visiting. There are FCC regulations against that. I was told by the second rep that they don't know for sure if we were tethering, but my usage fit the profile of someone who tethers.
I played dumb and said I didn't know what AT&T was talking about and said I use Pandora and Netflix all day. She said they would note the account and remove the flag and I shouldn't receive any further emails. I've still been tethering and have used almost 5G since that call and no notices from AT&T.
An interesting side-note... the AT&T rep explained that tethering was connecting your iPhone to any other device whether or not that devices transmits or receives data. She said you can't connect your iPhone to your TV to watch Netflix on a larger screen, you can't connect bluetooth devices. I asked about bluetooth headsets and iTunes. She said that iTunes is okay since you need to do that to update and sync your device. Bluetooth headsets they weren't worried about. Syncing your iPhone to iTunes over wifi, not okay. That's when I realized that AT&T's definition of tethering is all over the place. I read several other posts on this topic and it sounds like different reps are giving out all kinds of erroneous information. It could be that the reps don't know because AT&T isn't telling them either. If AT&T really knew who was tethering or not, they would just convert your plan and be done. Why the opportunity to call in? Oh yeah, that's an opportunity for the service rep to guilt you into changing plans.
Bottom line... continue tethering, call AT&T and let them know you are a high data user and have fun.