http://www.wired.com/gamelife/2010/03/steam-mac/
Now all you OSX users have no excuse. Play TF2 with us!
Highlights from the article:
If players already own the PC versions of Valve games, they’ll get Mac versions at no extra charge through a feature called Steam Play.
By using the Steam Cloud feature that the company introduced in 2008, players can save in-progress games online, then call up those saved games no matter which version they’re playing. If you’re playing Half-Life 2 on your home PC but then head out on the road with your MacBook, you can continue your game-in-progress.
Emphasis added.