Titled S.T.A.L.K.E.R. Clear Sky, the game is said to be a prequel to S.T.A.L.K.E.R.: Shadow of Chernobyl, fleshing out the story of one of the main characters of the first game. The game world will consist of a mix of familiar and new territory, with more anomalies and vastly different landscapes.
On the technical side of things, look for interface and PDA tweaks, along with overhauled AI.
Update: We now have screenshots.