I got a formal letter stating that "Our records show that you use this capability, but are not subscribed to our tethering plan." They further stated that if I stopped tethering, nothing would happen. The letter was dated April 6, 2011, and it also stated that if I tethered after April 16, 2011 they would change my unlimited data plan to a DataPro 4GB for Smartphone Tethering - $45 per month(this gives you 4GB in total, combining both your smartphone data plan for $25 and the tethering feature, $20)
I believe the key factor here for me was my latest bill online which shows 34,974.58 MB as of 4-17-2011.
The Cellular Network Data Usage record on my iPhone 3gs shows 18.4 GB sent and 524 GB received. I have never reset the statistics, so I can only assume that this number indicates my total data usage for the past year plus that I have had my iPhone.
Time Warner cable just installed broadband cable in my apartment with 10mbps down, for $29.95 per month, so there should be little need for me to consider tethering in the future, not that I admit to doing so in any event.