I got my iPhone 3GS brand new in 2009 during the height of the "Unlimited' data plans and it was just starting to change... I quickly Jailbroke and paid for MyFi or whatever and used to tether my ASUS G50VT-X5 a lot. A couple things, I used to like to watch JTV with it. When you tethered and watched JTV I would rack up like a GB of data in a few hours of watching. The week my Grandfather died in like may? 2010? I was at my mothers all week during the day, tethering , I think I did like 9 GB data just by tethering M-F 9ish to 5ish and some decent chunks of JTV streaming.
Now I never got bothered from AT&T for too much Data becasue apparently I used miniscule amounts of data for like 8 months of my contract viA HAving routers at both wortk and home, but then started tethering an used a lot in a short period of time--- BUT--- this was just the beggining of the famous tethering emails/text messages and warning about the 5 GB soft cap.
It was my understanding it was or shall say still is extremely easy for them to figure out if you're tethering and now its all contractual and pay-for-it-basis so on almost all the devices/carriers, if you're tethering and not paying them for it they bust you. I am not really familiar with the latest data plans bec. I got out of it, won't do the pay-for-data-buckets deal, but I think if you tether and not pay them for the data/tethering features they either charge you, suspend you, throttle you, etc. or cut you off, some combination etc. maybe.