Details can be found on the Valve Software web-site. They also have information on their Wiki as making free access to the Source SDK should allow their "modding" community to further flourish.
This announcement comes just a few days after Valve announced they would be making Team Fortress 2 available free of charge.
Those interested in checking out the Source SDK can obtain it from the Steam client, per long as they have purchased a Source Engine game from the multi-platform client like Counter-Strike: Source or Half-Life 2.
I'm afraid I don't have time to comment further or analyze the SDK at this time due to a Phoronix trip to Washington.