Kevin Drum has a terrific series of posts (1, 2, 3) on the ever-popular cap-and-trade vs. carbon tax debate. The short version is that, in the messy real world, a carbon tax won't likely be any more elegant than cap-and-trade once Congress through with it; both approaches will require hefty new regulations; there are reasons to prefer cap-and-trade (the flat cap obviates the need to constantly fiddle with tax rates to meet emissions targets); and—most significantly—cap-and-trade has a realistic chance of passing in the next few years, while there's virtually zero momentum in Congress behind a carbon tax.

P.S. Also see Dave Roberts's post on this topic. In addition to the above, he makes the (under-appreciated) point that, in the short term, complementary policies like a renewable-electricity standard and new energy-efficiency rules will likely do more to reduce emissions than either a carbon tax or cap-and-trade (which are more important for reducing emissions in the medium and long run), and that shouldn't get lost in the discussion. 

--Bradford Plumer