I know that at least AT&T isn't charging for this right now. In fact, my bill went DOWN when I switched from a 250mb/450minute/limited texts plan to a 1gb/unlim

I know that at least AT&T isn't charging for this right now. In fact, my bill went DOWN when I switched from a 250mb/450minute/limited texts plan to a 1gb/unlimited talk/texts plan that included tethering. The tethering isn't a separate item on the bill. But it's possible that older plans won't "support" it - I think that was the case with mine.