Does PC version of Civ-5 on Steam entitle me to Mac version?
Solution 1:
You can tell by the SteamPlay logo. The SteamPlay logo means that a game is playable on both Mac and PC, and just a Windows logo means it's only playable on Windows.
This is the SteamPlay logo: 
This is the Windows only logo: 
In other words, yes.
Solution 2:
If Civ 5 will be part of the Steam Play program, then, Yes you will be able to download the Mac version when it releases. (this is further reinforced by the experiences from Civ4)