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)
