Meh...I'm pretty sure they keep pushing it back in order to modify the engine for newer instruction sets, and if this game is a testiment to anything, I believe it will be the next breakthrough game in all aspects including gameplay, physics, graphics whatever
I heard it had been pushed to 2007 a few months ago I swear but oh well...HL2 was delayed well over a year and it's engine was still considered pinnacle of its time and it delivered in every way possible, I'm not going to hope on a date for stalker although I haven't after the last few delays almost a year ago...however I'm going to buy the game when it comes out and I'm sure it will be out at some point and will certainly be a game to remember
And yeah, it's not difficult to port or update an engine if source is any example of this with HDR