If you get teh PS3 version you get a code to unlock it on steam, for free.
You can not use your steam and PS3 version at the same time, so if i login to steam on ps3 i cant play portal 2 on pc until i log off my ps3.
And yes, PS3 now has steam support, so you can play from ps3 to PC. amazing.
i think ill get ps3 version. or maybe PC? i dont know now
~RQ