This page at boomtown reports that there will soon be an open multiplayer test of this much anticipated game.

A news post on THQ's official site in Germany announces an open beta test of S.T.A.L.K.E.R.'s multiplayer portion. 5000 gamers will get the chance to test the game's multiplayer action well before the official release in Q1 2005.

I also heard that a team of highly experienced Counter-Strike players has been employed on this game polishing the multiplayer portion of the game, could be interesting.