Aspyr didn't hold the digital distribution rights to Civ IV, so 2K was allowed to make the game SteamPlay. I'm sure Aspyr wasn't happy about it, and it's the only Aspyr product on Steam. They probably worked out a different licensing arrangement this time around.
Well if it isn't distributed on Steam, it will be useless for multiplayer. If they release it on Steam, it will most likely released on steamplay. If it's released on Steam play, it will have to be given to early PC purchasers since otherwise there is no framework to buy the Mac version for those users. So really it will either A. Not be on steam and not have steamworks multiplayer with Windows users, or B. be available for free to early Windows purchasers.