I got that one a while back when I was in Sacramento and used nothing but 3G to access internet on my phone for a whole weekend. I did not tether at all. They just assumed that since I used a lot more data than usual that I must have been tethering. I called and explained it and they told me that they would clear it up.
The text told me that if I did not respond that they would automatically switched my network plan (remove my grandfathered unlimited and add a tethering plan). Seemed like extortion to me