Valve has confirmed how the Steam elements of the PS3 version of Portal 2 are going to work, saying the game will provide cross-platform matchmaking, gameplay, friends, chat, and achievements.
The PS3 version of Portal 2 is to have a Steam overlay, presumably in the same vein as the one you're familiar with on PC.
The overlay gives access to both PSN and Steam friends. It will display their in-game or online status and providing a pathway to text chats, player profiles, friend requests, and game invites.
Steam Achievements are earned in lockstep with PSN Trophies and are also accessible within the Steam overlay.
The game will be Steam Cloud-enabled, and will save both both co-op game progress and single-player play remotely, meaning you'll be able to pick up and carry on from any PS3 you like.
The PS3 version of Steam will require you to link your PSN account with a Steam account, which is done within the game.
A Steam user can login to an existing account, and anyone new to Steam can create an account easily.
As previously reported linked players of the PS3 version of Portal 2 can also access and play their copy of Portal 2 on any PC or Mac by redeeming an in-box access code on their computer.
The Steam client verifies the linked customer and then lists Portal 2 among their desktop Steam library.
None of this functionality will be available in the 360 version. Portal 2 releases next week.