Attention! Valve has announced exactly how Portal 2‘s Steam features will work on the PS3. PS3 gamers can look forward to cross-platform (with the PC) matchmaking and gameplay, real-time chat, Steam achievements, and access to the Steam Cloud for game saves.
PS3 gamers will have to link their PSN account to the Steam account. If you don’t have a Steam account you can create one right from inside Portal 2.
The Steam overlay is there, too. And let’s not forget that the PS3 copy comes with a code to play on your PC or Mac, too.
In short, it’s Steam, but on your PS3. Not hard to figure out.
Valve has a big FAQ here if you’ve any further questions about how this whole thing works.