Drum responds to my quibble and to Michael O'Hare. The most cogent piece of Kevin's argument to my mind is the following:

The carbon exchange itself, of course, does need to be set up and kept in operation by a government agency.  That's extra work compared to a tax, and it has to be done right.  Still, this is hardly untrod territory.  There are hundreds of electronic commodity exchanges around the world and we know how to set one up.  In fact, we've done it before for other cap-and-trade programs, and the operation of the exchange itself has never been that big a deal ...

O'Hare's right that cap-and-trade is no cheaper than a tax (and it would be dishonest to imply otherwise), but I think he's wrong to believe that setting the proper tax level is easier and more efficient than setting the cap level directly.  From the point of view of both politics and public support, I think it's exactly the opposite.