Today there is a informative S.T.A.L.K.E.R.: Oblivion Lost interview posted at BGN.be. There at BGN you will also find a new screen from the game. Here is a clip from the interview:

BGN: Will STALKER have elements of RPG or Adventure games ?

Oleg: Yes, both of these. We define the game as an adventure 3D action with elements of RPG and tactics. For example, player can collect artefacts and exchange them for weapons and equipment, and he is also to explore the constantly changing anomalous Zone, communicating with Zone creatures and other stalkers in the area.