In case the actual statute isn't enough, here's more on why it's a crime

Everyone else is reprinting this, so I'll join the party.

Former Sen. Gary Hart, at HuffPo (emphases mine):

It is now fashionable among columnists supporting the Bush administration, New York Times journalist Judith Miller, Robert Novak and the increasing network of senior administration officials implicated in the Valerie Plame Wilson outing to say, "So what? Where's the crime?"

The federal statute making it a criminal penalty to knowingly divulge the identity of anyone working undercover for the Central Intelligence Agency was not enacted in a vacuum.

In the early 1970s, in part as a result of the radicalization of individuals and groups over the Vietnam War, a former CIA employee named Philip Agee wrote a book revealing the identities of several dozen CIA employees, many under deep cover and some including agency station chiefs in foreign capitals.

Many of the countries in which those CIA employees were working themselves had extremely radical and violent elements stirred to hatred over their opposition to America's conduct in the Vietnam War. So, by revealing their identities, Agee had knowingly and willingly placed these American citizens at risk. Violent consequences were predictable.

Richard Welch, a brilliant Harvard-educated classicist, had been stationed in Greece as CIA station chief only a few months before he was murdered, by a radical Greek terrorist organization called the 17th of November, in the doorway of his house in Athens on Dec. 23, 1975. Had Agee not divulged his name, there is every reason to believe that Welch would be alive today after decades of loyal service to his country.

Largely as a result of Agee's perfidy and Welch's unnecessary death, the Intelligence Identities Protection Act (IIPA) of 1982 was enacted, making it a felony to knowingly divulge the identity of a covert CIA operative. It carries penalties of 10 years in prison and a $50,000 fine for each offense. There are those who dismiss the crime by saying, "Oh, Wilson only had a desk job." That is not a defense under this felony statute. It is for the CIA, not Karl Rove or Robert Novak, to determine who requires identity protection and who does not.

The political irony of all this is that conservative elements in America have always proclaimed themselves more concerned than anyone else with national security, the sanctity of classified information, protection of sources, support for our intelligence and military services, and so on. At radical times in our past, irresponsible leftist groups thought it was their duty to try to reveal the names of CIA agents. Now, under a conservative administration, it is these conservative national security champions who are saying, with regard to the "outing" of a CIA undercover officer, "Where's the crime?"

There is further irony in the fact that now the premier intelligence agency of the United States, the CIA, is in utter disarray. Morale is desperately low. Many of the best career officers are leaving. As the source of unbiased professional intelligence, the CIA has been diminished and pushed aside by the Department of Defense. This at a time when it is critical to national security to have the best possible intelligence to protect us from terrorism.

I served on the first Senate Intelligence Oversight Committee in the late 1970s and have continued to be a strong believer in and supporter of the CIA. I deplore those who want to diminish it, politicize it, or require it to produce bogus intelligence it would not otherwise produce simply to fit some preconceived political or ideological agenda. In almost every case where the CIA has malfunctioned, it did so under pressure from one political administration or another.

So, there's the crime. To casually and willfully endanger the life of an undercover CIA agent is a felony. You either believe in taking the laws of the United States seriously or you do not. Citizens - even highly placed ones - do not get to pick and choose which laws they will obey and which they will not. Miller and her publisher may think she's a hero, but I don't. It is well established that there is no First Amendment protection for a journalist or anyone else to withhold evidence of a crime.

There is one final irony to this story. On Christmas Eve in 1975, I got a call at my home from the director of the CIA, William Colby. He asked if I would intervene with the White House to obtain presidential approval to have Welch buried at Arlington National Cemetery, a hero fallen in service to his country. I quickly called President Ford's chief of staff on Colby's behalf and made the request. Within two hours, the president had agreed to sign the order permitting Welch to be buried at Arlington.

The chief of staff's name was Richard Cheney.

Update: I incorrectly stated above that Philip Agee included the name of Richard Welch in his book naming CIA operatives. That statement was inaccurate. Mr. Agee did not identify Richard Welch, but other sources did. Nevertheless, the Agee book and subsequent Agee actions did contribute substantially to the passage of the Intelligence Identities Protection Act of 1982. I apologize to Mr. Agee for this incorrect assertion.