A recent post by community manager Don Reba over at GSC Game World's official forums suggests the developer may be looking to take its open world post-apocalyptic FPS into the realms of the massively multiplayer.
From the post:
2. Will there be S.T.A.L.K.E.R. Online?
We do not quite understand what you mean by "online". If it is the cooperative mode, then no, there will be none. Even at the time of development of the first part of the game we said that a cooperative mode is impossible, since Life Simulation would be different at each computer. Synchronizing it would be practically not doable. If you mean an MMOG, then we are seriously considering it.
I'm all for a massively multiplayer STALKER game - heck, it would probably be a lot better than Interplay's current attempt to transform Fallout into an MMO. But considering THQ had to bring in Dean Sharpe to help GSC Game World get STALKER done after an extremely prolonged development cycle (albeit not as bad as DNF), I wouldn't expect anything to appear for at least a decade.
There's also a short bit in the FAQ confirming Direct3D 10 support in the new version of GSC's X-Ray engine. It won't make it into STALKER, but it'll definitely be in the developer's next game.