Since Mac BtS is still a rumour, you'll need to wait for official word on the subject from Aspyr. However, as BtS uses the same game software as Warlords and Civ4, it's reasonable to assume that it will share their ability to play cross platform network games.
Note that there have been reports of "loss of sync" problems in these forums, and you don't have to dredge up a three year old thread to find them! However, I've seen similar reports when PC-PC players try network play, as well.
There is at least one unofficial "patch" that claims to reduce the Windows sync problems. I think it's a mod, using a custom DLL. If so, this will not run on the Mac version until we get a way to recompile custom DLLs.