Thanks metal, that did actually clear it up for me somewhat (and Dean's right about what I was really trying to ask).
Based on what metal posted, I imagine that the PS3 copy will come with a code that lets you tie your PSN account to your Steam account, or if anyone can do that then the code will be what lets you activate your PS3 copy of Portal 2 on Steam. Then probably anyone will be able to play the PS3 version of Portal 2 as long as they have the disc, but only people who've "activated" it with the code will be able to use the Steam features, whatever those might be (probably cross-platform coop, for one).
This is all just my conjecture, though.