Right from the horses mouth…:

Steam on the Mac will behave just like Steam for Windows, but through Steam Play will add cross-platform ownership. That means if you own Left 4 Dead 2 for Windows, then you already own it for the Mac. There’s no need to buy it twice! Unless you want to buy it twice, in which case, as long as you’re feeling so generous, why not buy it five times?

Plus, Mac and Windows players can go head-to-head, since both Left 4 Deads are cross-platform compatible. Mac owners can even host their own servers and create their own maps, just like Windows users.

The Mac Versions of L4D1 and 2 don’t run under emulation software but in native code. That’s technical talk for how cool your friends will think you are with your five copies of Left 4 Dead 2 for the Mac.

We’ll be releasing the exact system requirements soon. The Mac versions will be updated in sync with the Windows versions, so the platform wars can continue uninterrupted.