Those of you who use Steam will know that SteamPlay basically means that when you buy the PC version of a game, you also get access to the Mac version (and vice versa). Apparently, starting with Portal 2, buying the PS3 version of a SteamPlay game will grant you access to the PC and Mac versions too. Looks like the PS3 version of the game will be the one to go for even if you primarily intend to play the game on PC.
It obviously also supports other SteamWorks features like cross-platform play, voice chat etc.
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