If you play Portal 2 on PS3, you can play with people on a PC. Neat! And to make sure as many people as possible are doing just that, the PS3 version of the game will include the PC version.
All players have to do once they get the game is link their PlayStation Network and Steam accounts. Once that's done, they'll have a Steam Play (so, PC and/or Mac) version of the game ready to download in their library list.
This cross-platform play even extends to giving PS3 users the ability to chat to PC gamers, and vice-versa. Oh, and PS3 users can even save their game in the Steam Cloud, meaning Valve has them tucked away in their servers in case something ever goes horribly wrong with your PS3.
That's a whole lot of "firsts" for both Steam, the PC multiplayer service of choice, and Sony's PlayStation Network. Not to mention a free copy of the game you can, for example, take with you on a laptop when you're not on your couch with the PS3.
And if all this works, and works well, let us welcome out glorious cross-platform future with open arms!