Downloadable add-ons include packs featuring the Mediterranean, Mesopotamia, Asia, and the Americas for $2.99 each or $9.99 as a bundle. The Babylon Civilization pack and Spain and Inca pack are available for $4.99 and $7.99, respectively. The Mongol DLC pack is also available, albeit for free, and "will automatically download upon launch of the Steam client in Online mode."
The premium downloadable content was first offered as pre-order incentives from multiple retailers before arriving for purchase on PC in December. Civilization V launched in November, two months following its release on PC.
I hope this means that the Mac version has been updated to be compatible with the PC version. It had been 2 months since the last Mac patch and the PC version was patched only a week later, breaking compatibility and providing a number of game-changing tweaks. Since I bought it, I've been a little dissatisfied with not having the latest patch, but I'm looking forward to checking it out again when I get home.
Did cross platform play work when both releases were at the same version? I thought I'd heard reports contrary to that.