I'm outsourcing this post to Dean Baker. Summary: refineries are the choke point in gasoline production, so they can easily raise prices if the gas tax is lifted. No one else will get the benefit.
UPDATE: I think this is a corollary of a discussion at Gristmill a while back, to the effect that a modest tax on oil imports wouldn't really raise prices and reduce consumption but rather would claw back some profits from overseas producers. In that case, the environmental value isn't so much in the existence of the tax but in what could be done with it.