So this recent blog post on l4d.com answers some questions surrounding Steam for Mac

Namely,

• Game ownership is cross-platform. If you own it for Windows, you own it for Mac, and vice versa.
• Cross-platform gameplay. You can play with Windows users. All updates for L4D/L4D2 are to launch "In sync" with Windows versions.
• Mac Versions of L4D1/L4D2 aren't ported but run in "Native code".
• Map creation and dedicated server operation will be available to mac users.

Still a few questions such as release date and system requirements, but hopefully this dispels some fears surrounding Steam for Mac.

Bah, this is basically a repost. I only searched the PC forum and found nothing, but it was posted earlier by PlayStation Gamer in gaming.

mods please lock.