"It was Holtz-Eakin who made the point that beneath the surface of cap-and-trade is a broad philosophical difference. McCain's opponents, he said, would not stop with cap-and-trade. They would supplement it with additional policies and regulations: increases in CAFE, efficiency standards, renewable portfolio standards, etc. This, Holtz-Eakin contended, was a symptom of their addiction to the heavy hand of government control, used to "micromanage" the economy. McCain, he said, would pass cap-and-trade and otherwise get out of the way and let the economy do its thing.
Grumet made the point that to get the reductions we need purely with cap-and-trade, you'd need a price per ton of carbon so high as to be prohibitive -- on the order of $150/ton. A more reasonable price, say $35, wouldn't do much to affect, say, the transportation sector. That is precisely why you need CAFE and other supplemental policies (efficiency standards, utility decoupling, etc.).
(For what it's worth, as I understand it Grumet has the better of this one. Most energy policy analysts I know of recognize that a reasonable price on carbon is only the beginning, and will not in and of itself get us where we need to go.)"
http://gristmill.grist.org/story/2008/3/14/221425/110/