With a small update to the official website, Ritual un-officially announced that the long awaited SiN Episodes launch would now be in early 2006. The first episode titled Emergence was previously scheduled to be released sometime in the Fall of 2005. SiN Episodes is being developed on the Source engine and distributed on the Steam content delivery system.
While one can make assumptions on the delay, also remember that the Half-Life 2 expansion Aftermath has also been delayed until early 2006. Coincidence? Perhaps so, but remember Valve is always adding and fixing the Source engine. It was only just announced that Day of Defeat: Source would be getting some new features in an upcoming patch such as Depth of Field, Motion Blur, Dust and Color Correction and Film Grain. Maybe Ritual decided to hold off on a release until these features are ready for primetime so that they could take advantage of them?
News source: SiN Episodes Homepage