Here is a snippet from an Interview over at Grist (go over and read the whole thing, he acquits himself quite well):
"Q: Do you believe that we need a carbon tax in addition to a cap-and-trade program?"
"Obama: I believe that, depending on how it is designed, a carbon tax accomplishes much of the same thing that a cap-and-trade program accomplishes. The danger in a cap-and-trade system is that the permits to emit greenhouse gases are given away for free as opposed to priced at auction. One of the mistakes the Europeans made in setting up a cap-and-trade system was to give too many of those permits away. So as I roll out my proposals for a cap-and-trade system, I will price permits so that it has much of the same effect as a carbon tax."
He makes an important point that I have been repeating ad nauseaum, you don't really have a cap in a cap and trade unless you auction permits. A good follow-up question would have been what Obama would intend to do with the resulting windfall.