I cannot recall a media blackout this massive ever having been implemented on a US military computer network. This is unprecedented.
Reuters: "The U.S. Air Force has blocked employees from visiting media websites carrying leaked WikiLeaks documents, including The New York Times and the Guardian, a spokesman said on Tuesday."
The Wall Street Journal broke the story here (paywall link).
Eric Schmitt in the New York Times reports that more than 25 media websites are now blocked for this reason:
When Air Force personnel on the service's computer network try to view the Web sites of The Times, the British newspaper The Guardian, the German magazine Der Spiegel, the Spanish newspaper El País and the French newspaper Le Monde, as well as other sites that posted full confidential cables, the screen says "Access Denied: Internet usage is logged and monitored," according to an Air Force official whose access was blocked and who shared the screen warning with The Times. Violators are warned that they face punishment if they try to view classified material from unauthorized Web sites.