I occasionally used MyWi on my iPhone 4 when I had it...Seldomly connected my iPad on travel and would use it in stores that didn't have WiFi and I wanted to log on to a computer to check things (looking at you Frys & Best Buy!!!).
I never had any texts or notices from AT&T about this use.
Then, in July of 2012, I was helping a coworker out that had no internet and tethered their laptop via USB. Didn't even have that good of a signal, just turned on MyWi for about 5 minutes or so. That evening I got the "free text" stating that I was not on a tethering plan and they were going to switch me into one if I continued to tether without paying them the required $50/mo for 5gb of data tethering plan.
I called them up, plead my case that I have unlimited data it shouldn't matter what I do with it, they stated I was using my phone in an unauthorized manner and they could cancel my plan. I said I would no longer tether and to NOT upgrade my plan with the $50 additional per month tethering plan.
I have not tethered since. It didn't matter much to me anyway as I got an iPhone 5 in September and haven't been able to tether until the phone was jailbroken.
Since then anyway, I've gone another route and have a 4G USB Verizon card with unlimited useage as I refuse to give AT&T another cent. Especially since I am still unlimited yet "throttled" at 5GB on LTE!