Matthew Yglesias points out an argument from Ann Althouse that's as pathetic as it is absurd. Overlooked, though, are the deeper absurdities of the statistic first brought up by Orin Kerr: "Today the war in Iraq passed a very sad milestone: the number of U.S. troops killed in Iraq has now exceeded the number of persons killed in the 9/11 attacks."
The appropriate response to this is to point out that it is of no consequence whatever. Or, "Uh, who cares?"
2,973 isn't a magic number, or the minimum number of people you have to kill to have done something wrong or regrettable. It's just the number of people who happened to have died in the attacks of September 11, 2001. One (1) dead person is a large moral problem; 2,973 of them is not a different sort of problem. What we're dealing with is a numerical coincidence, not a normative threshold of any significance.
P.S. - I'd sort of hoped that Althouse would explain in the comments to her post what she meant when she described invading Iraq as "fighting back". No such luck.