In the latest community Q&A with S.T.A.L.K.E.R developer GSC Game World they say that the STALKER: Shadow of Chernobyl singleplayer SDK can't be released now but maybe sometime later, DX8 renderer will be supported in Clear Sky, "the player will meet characters he knows" and more.
Question: Will there be S.T.A.L.K.E.R. Online?
GSC Game World: 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.