PC gaming, as the name suggests, is usually something exclusive to the PC. Sometimes there are Mac ports, but any serious computer gamer would either have a dedicated gaming PC or at the very least a Boot Camp partition.
Steam, the online service for PC gamers developed by Valve, looks to soon be expanding onto Mac OS X. While there hasn't yet been an official announcement (perhaps at the Game Developers Conference next week), Valve has sent various teaser images that, when put together, point squarely at a product or service made for the Mac.
Educated guesses would say that all of Valve's internally-developed properties will be available for Mac, which appear to include Steam, Portal, Team Fortress 2, the Half-Life series and the Left 4 Dead games.
With Valve now pushing Steam and games on Steam for Macs, and Blizzard releasing its titles for Macs, the two companies could encourage releases for Macs in a very big way. If this continues, a wave of games that are produced for Mac and PC at the same time, could eventually change the Mac platform forever.