Official Penumbra: Black Plague Release Date: February 12, 2008

[WindexGlow]

Chances are, the linux/mac versions won't be available at release. But you can expect, are these.
1: Members talking about how fun the game was
2: Extremely high reviews of the game
3: People feeling bad that others have to wait to play it
4: The mac version coming within a month, and Linux at 1-3 months

And the last one is just a guess. It took them many more months to get mac/linux out, but now that they know what to do it should be much faster.