Valve sent over a detailed FAQ regarding how Steam will work for the Playstation 3 when Portal 2 releases next week.
Portal 2 for PS3 will be the only console to utilize cross-platform play between PSN and Steam users and grants such functionality like “matchmaking, gameplay, friends, chat, and achievements” through the Steam API. The Steam presentation for PS3 will provide in-game and online status updates with access to “text chats, player profiles, friend requests, and game invites.”
Steam Achievements will be unlocked in conjunction with PSN trophies. Steam on PS3 will work once the player has linked their PSN account with a Steam account, which users can register right in-game. Players will also receive access to a playable copy of Portal 2 for PC through an access code found in retail boxes.