I think everyone in this thread is going to be disappointed -- you all seem to be forgetting that Aspyr (not the original Civ 4 developers) did the Mac port of the game, and it's sold completely separately from the PC version. This is why the Mac version is significantly more expensive than the PC version, and also why it makes no sense to be able to buy the PC version and then download the Mac version through Steam -- Aspyr doesn't get a dime from somebody buying the PC version, only the Mac version. I think it's unlikely that Steam Play will ever work for Civ 4 the way it does for PC exclusives.
Right now Beyond the Sword for Mac is selling for $35 on Amazon, while the PC version is selling for $10. Neither Aspyr nor 2K Games has any economic incentive to let you buy the cheaper one and then download the more expensive one for free, as opposed to the case of Valve games, where both the PC and the Mac versions end up as revenue for Valve.