chiefie: You will need to register for Steam account, and I think the Steam code for PC/Mac is tied to your PS3, as it's one and same, you can't play on PS3 and on PC at the same time..
I could be wrong, it could just be a game voucher but I won't imagine Valve go all the way with brining SteamWorks for PS3 and not making it single same "game" across multiple platform.
Video below on how to redeem, sounds like it still uses a code for PC/Mac. With a bit of mucking around using different PS3 profiles you might be able to activate the code on a mates steam account.
http://www.joystiq.com/2011/04/19/portal-2-on-ps3-how-to-redeem-the-free-steam-copy-for-your-pc/

