As some people may, or may not know, Valve released a Mac version of Steam a few weeks back. Already there are plans for many games to be cross-platform. Civilization V is a good example of this - you buy it once, and you can install it on mac or windows, and Multiplayer will work for anyone who owns the game, regardless of platform.
What do you think the chances are that CA will release the game for both Windows and Mac, like what is being done with Civ V?
Personally, i think its unlikely, but since my computer is a mac, i'm keeping my fingers crossed, if not for Shogun II, then hopefully for the next title.
Edit: Seeing as i wrote this post at 4 in the morning, it actually sliped my mind - Rome, Barbarian Invasion and Alexander were all released for Mac, and they all play quite well too.
It took the company a fairly long time to translate the code, but i believe it would be easier and faster to do if they did it while writing it in the first place.