Bluetooth pairing and tethering are sort of two different concepts. Yes, you can tether over a Bluetooth pair, but you can also tether over USB or wifi and you can also use a Bluetooth pair for other things like object exchange and presence detection. Tethering is prohibited by AT&T on the iPhone data plans (and all other data plans except for one Blackberry with tethering plan, AFAIK), but AT&T generally leaves Bluetooth pairing for other purposes unmolested (unlike Verizon, who always cripples it to push their stupid syncing services). My guess is Apple crippled Bluetooth (to mono headsets only) on the iPhone so they could push their MobileMe service for wireless contact/calendar/email syncing.
AFAIK there are apps out their for jailbroken iPhones to tether via wifi, but not via Bluetooth.