When the guys working at the Steam office get off their backsides and flip a switch / do whatever they need to do to release the DLC and not before.
It is currently 12:02 PM PDT in Washington State where Valve (who operate Steam) are based.
Where I am in Australia it's 6 AM on the 26th - go figure
So you see it's really the fault of you silly Americans and your silly backwards time
