In much the same way that Elder Scrolls games are like Everquest without stupid people, S.T.A.L.K.E.R. sort of reminded us of Anarchy Online without the crushing, ever-present stench of failure. It seems that the similarities to an MMO have not been lost on the creators of the game, who chatted about the possibilities in a recent Q&A with fans.
When asked if S.T.A.L.K.E.R. Online was in the works, GSC Game World shot down the idea of an online co-op version of the game, but said "If you mean an MMOG, then we are seriously considering it," which should definitely be encouraging news for fans. If GSC goes from "seriously considering" to "seriously announcing" it, we'll be sure to let you know.