Steam+Linux - Would I need to buy one or two copies?

[pxc]

As this is my first post, I'll introduce myself:

Hi, I'm Patrick!

--

Anyhow, today I heard about Amnesia from a friend, who saw it on sale via Steam. Since it's gotten rave reviews and the price is favorable, I really want to buy it. However, I need to know something about the licensing system.

If I buy a copy of the game on Steam, will I also be able to use the Linux version? Steam has a feature for some third-party games (Crysis is the example that I own) that allows you to see what "CD Key" your game has, independent from Steam. From the library, you can right click on the game and select "View Game CD Key". If I were to buy the game via Steam, could I use this feature to enable the Linux client as well? Alternatively, if I purchase the game from your store, will I receive a CD key that I can use for the Linux client and also register with Steam for the Windows version?

(As an aside, has anyone tried your Linux client on BSD?)