And at the risk of pissing off some decent people, I'll add one other thing. In the near term, no serious carbon tax will ever pass the U.S. Senate. Period. If you believe otherwise, you're just not paying attention to things. A big part of the surge in interest in a carbon tax is purely cynical, coming from special interests who are afraid a carbon cap might actually pass and want to muddy the waters with pseudo-liberal arguments in order to build an anti-C&T alliance and keep anything at all from passing. There are plenty of carbon tax advocates who are perfectly sincere, but I gotta tell them: you're being played by people who are the farthest thing imaginable from sincere. If you win, we're not going to get a carbon tax. We're going to get nothing.
Barring a sea change in the political situations (possibly brought on by a literal sea change?) cap-and-trade is our best bet to get some legislation passed that could have a positive impact.
It's worth checking out Drum's primer on cap and trade from April. I'm not swayed by all points, but it's definitely more realistic than the carbon tax.