I've had tethering for years from t-mobile in the US. I've had at least two different plans over that time, and I don't recall ever having to explicitly request it or pay extra to get it.
Like all the other US providers, plans change frequently, very few have unlimited plans, and none that I know of have unlimited-speed unlimited-data tethering plans.
Currently, for $45, the best you can get on T-Mobile is 4GB of unlimited-speed tethered data, and for $70 you can get 512KB/s tethering. The cheapest mostly-unlimited data plan (including fewer limits for tethering) for one line costs $95. Older plans, which had cost less and had less limits, have been mostly phased out.
Another sad irony is to get the cheaper plan, you have to get a credit check, which dents your credit score, just to acquire a "pre-paid" plan. Also, T-Mobile's website seems to have been scrubbed of their prepaid plans, so probably you can only find details via interrogating their salespeople.
In 2006~ish, I think I had to pay t-mobile about $10-20/mo to be able to tether my Moto Razr at fairly low speeds but unlimited data. By the time I jailbreak'd an iPhone for use on the network, I didn't have to pay anything extra beyond the plan data, so it's been quite a long time.
