Cross-platform matchmaking will soon be possible with the release of Portal 2.
Valve recently detailed their plans for Steam on the PS3 and announced that the Sony console's version of Portal 2 will be fully integrated into the digital hub. According to Valve, Portal 2 on the PS3 "offers cross-platform matchmaking, gameplay, friends, chat, and achievements, all powered by Steam. Steam on the PS3 also saves co-op game progress and single player saved games to the Steam Cloud, enabling players to pick up where they left off on any PlayStation 3."
Steam on the PS3 will also give users "access to both PSN and Steam friends, displaying their in-game or online status and providing a quick pathway to text chats, player profiles, friend requests, and game invites."
Finally, the PS3 version of Portal 2 will include a code for a free PC version of the game, although that's not exactly new news. It's still pretty exciting, and it should be interesting to how the PS3/Steam relationship will grow with other games and continued cross-platform support.