Aiming for simultaneous release with PC, but might not make it.
Game will be SteamPlay, buy once play on multiple platforms. They suggest that people wanting to play it on Mac buy it now, if they want, just buy it on a Mac (presumably this affects the platform metrics)
There are system requirements, which have also been added to the pinned system requirements thread in this forum
There will be cross-platform MP, but possibly not as soon as the Mac version comes out, and may be broken by updates in future as well.
It will also be available on the Mac store, though cross-platform MP won’t be available on that version (if Civ5 is anything to go by, there may be no internet MP at all on that version)
UPDATE: There seems to be a preload for the Mac, but it’s very small, and Aspyr-Blair has said they aren’t doing a preload for Mac, so…