I'm not sure they will, because the Mac-compatible version of the core game is already for sale and available for download (and for twice the price of the game and all expansions for Windows only). I hope you're right, though.
EDIT: Looks like you are indeed correct, Bribo! Over at the Steam forums, they're saying it will be available and a bug is preventing those who newly purchased it with SteamPlay from installing it on the Mac side. Hooray!