it's not even disabled on AT&T, you just have to pay extra for it.

Actually, even that isn't entirely accurate anymore. Apparently tethering is included in the new 4GB data plan (which is $45, to the 2GB plan's $25).

exactly. I've got an ancient (2nd gen) iPhone on which I spend about $30 prepaid and it's been tethering fine for years. I know, because I use the tethering regularly

Curious, I often bring an iPhone to the USA and put a prepaid SIM in it when I get there, last few times I got an AT&T SIM, the tethering option vanished from my preferences. With a TMobile SIM, I can tether, but only at EDGE data rates, not 3G. (Last trip, I use AT&T in the iPad and tMobile in the phone, seemed to be my most cost effective set of options)

TMobile uses different frequencies, which is why you'll only get EDGE data rates with that TMobile SIM.

Can't help, sorry. My experience is with Telstra in Australia