I'm not really sure. We actually have a Linux version working, and in fact we use a modified version of the Linux client to run part of the game's AI. So I'm sure there will be a Linux version, because we've already done a lot of the work for it.
But the past few versions of the Unity middleware has had a lot of serious Linux bugs, with graphics, GUI input, keyboard focus, all kinds of things that are too low-level for us to work around. A bunch of those issues were fixed recently, so I'll test it out again in a few weeks and see how it's working. I know there are at least a few remaining bugs with text-entry to fix, so it will definitely still be buggy, but it may be "good enough" to be worth showing off.
If it's still very buggy and we can't work around the issues, though, we'll hold off on it until Unity 5 comes out.