Steam has been a giant on the PC for years, yet the announcement of the Steam platform coming to Mac OS X is considered "the biggest event in Steam's history" by John Cook, the Director of Steam Development. In an interview with Joystiq, Cook explained how cross PC and Mac game ownership will work, saying,
"This is supported by "Steam Play." We are using this feature to provide cross-platform access to all Valve games to those who already own the PC version. We hope other developers and publishers will use it in the same way."
Good news for Mac owners who already buy games on Steam on a PC, this writer included. Joystiq also confirmed with Cook that Apple is quite involved in Steam coming to Mac.