Valve explains Steam on PS3

Back in January (and even before that at E3,) it was announced that Portal 2 on the PS3 would bring Steam support to the platform.
Valve have now released a few more details about how that will work. Within Portal 2, PS3 users will be able to link the game to either an existing Steam account, or a new one they’ve just signed up for. Once the two are linked, it will provide cross-platform matchmaking and display the status of your friends on both Steam and PlayStation Network.

Steam achievements and PSN trophies will be earned “in lockstep,” and both single player and co-op progress can be saved to the ‘Steam cloud’ service, allowing people to pick up where they left off on any PS3.
In addition, the PS3 version of Portal 2 will come with an in-box code allowing the buyer to redeem a PC/Mac version of their game on Steam at no extra cost. In doing this, Valve is eroding the concept of a single game on different systems requiring multiple purchases.
A detailed FAQ can be read here.