I just finished updating the OS to iOs 4, also using AT&T service.
Yes, tethering option will be available, but when you click on the button to configure it, it asked you to contact AT&T to joing the plan... That is, Tethering is an extra $20/mo on top of the unlimited data plan of $30/mo...
So yes, it's a pretty bad deal, even if you have an unlimited data plan.
BTW, can anyone point me to some places in Kaohsiung area that can help me with JB/unlock the Locked 3Gs (AT&T) with iOs 4? Thanks.