There’s much more detail here and here, but the gist of it is-

Joseph Wilson was a former US ambassador to Gabon who was asked by the CIA to go to Niger and investigate claims that Saddam was trying to source Yellowcake Uranium ore from the country. He concluded that the evil moustache wasn’t shopping there, filed his report and thought no more of it. Until Wubble U claimed the exact opposite in the State of the Union speech that was part of the pitch for invading Iraq. Wilson trashed the claims, starting a well deserved political outcry.

Someone in the White House decided that revenge, and possibly a warning to future critics, was needed. So they leaked to a right wing columnist that Wilson’s wife was an undercover CIA operative who had suggested the former ambassador be sent on the Niger mission. The columnist duly used this information in a piece trying to write off Wilson’s claims. This may not seem like much at first look, but think about it. His wife’s role had nothing to do with the validity of Wilson’s argument, and naming her has merely damaged her career and put her contacts at risk. And if anything, the revelation hasn’t had the desired effect. Rather, it has made plain (again) just how malicious and stupid the Bush administration is. Sooner or later something is going to sink the idiot government, preferably sooner.