The Mac version is out now (came out 23rd when Heavensward launched) and can be purchased from the SE store in your region. I don't know if it'll be coming to Steam, but just to clarify (in case you wondered) because you have the Steam version of the game with Windows doesn't tie you into buying the Mac version from Steam (only the Windows version has that issue in regards to expansions - Mac is treated as a different platform).

You do have to repurchase it AND the expansion (it comes as a bundle for Mac) so like when someone wants to play on PS3 or PS4 and they have to buy it again soon. However you can continue you playing with your existing subscription and you do get an extra 30 days free for buying the game.

However, you might want to read some of the user reviews first as there seem to be quite a few complaints about the Mac version and how it was implemented (essentially a wrapper over the Windows version which leads to considerably lower performance than if you just run the Windows version in Bootcamp):

http://forum.square-enix.com/ffxiv/t...e-reborn..-%28

(there are other threads, but the lead developer has posted in that one about it).