First off all, Portal 2 will be the first console game to debut Steam, and it will feature cross platform play in multiplayer games, cloud-based storage for saves games, and cross platform chat. Both cross platform features include the PS3 and PC/Mac.
Valve doesnt stop there either, because if you buy the PS3 version of Portal 2 you can unlock a Steam Play (PC/Mac) copy of the game for free by linking both your Steam and PSN accounts.
Valve president Gabe Newell had this to say about the PS3 version of Portal 2
“We made a promise to gamers at E3 that Portal 2 for the PlayStation 3 would be the best console version of the product. Working together with Sony we have identified a set of features we believe are very compelling to gamers. We hope to expand upon the foundation being laid in Portal 2 with more Steam features and functionality in DLC and future content releases.”
“We designed the Portal 2 PS3 experience to be very straightforward for gamers, said Josh Weier, project lead on Portal 2 at Valve. PS3 gamers will be able to simply drop the Blu-Ray disc in the PS3, link to their Steam account from inside the game, and all their Steam friends (on PC and Mac) will be visible and accessible for chat and game invites.”
source: Game on Smash