Gameinformer has now confirmed that rumors of S.T.A.L.K.E.R. 2 being cancelled were true. With this announcement, GSC has also been shut down. GSC Game World stated reasons why such as, "We did not reach agreement with the rights owner regarding the use of S.T.A.L.K.E.R. brand." Members of the team have now formed Vostok Games and are working on Survarium; a free to play MMO.
Here's the Gameinformer post if you want to see more info on this story. http://www.gameinformer.com/b/news/archive/2012/04/25/s-t-a-l-k-e-r-2-cancelled.aspx