Klei Entertainment's Mark of the Ninja is now live on Steam, completely in the open and totally unguarded against online purchases of $14.99. As for the trailer, it certainly isn't helping the 2D stealth game stay undetected.
Incidentally, Klei isn't resting its laurels after sending Mark of the Ninja on its way. The developer's next project is Don't Starve, described as "an uncompromising wilderness survival game full of science and magic." So, a bit like Half-Life 2 then? Oh wait, we've just seen the video, never mind.