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Valve just put out a press release, including this information:

Steam on the PS3 provides access to all of these features once the player has linked their PlayStation Network account with a Steam account, which is done right within the game. A Steam user can login to an existing account, and anyone new to Steam can create an account with the click of a button. In the spirit of Steam Play, linked players of the PS3 version of Portal 2 can also access and play their copy of Portal 2 on any PC or Mac by redeeming an in-box access code on their computer. The Steam client verifies the linked customer and then lists Portal 2 among their library of Steam games for PC and Mac.

So, it sounds like you need to have a PS3 connected to the internet, and to pair your PSN account with your Steam account (with Portal 2 in the PS3). Once you have a Steam account that is linked with a PSN account, you register the single use serial on the PC version of Steam to get Portal 2.

This indicates the following:
Two people can play co-op together (One on PC/Mac, one on PS3) with just one copy of the game, as long as you used a different PSN account to the one paired with the Steam account being used.
You will need to own (or have access to) a PS3 to be able to register for the free PC version.
Once you have registered the PC/Mac version, there shouldn’t be anything stopping you trading in the PS3 version.

You can get the PS3 version for £29.99 delivered here.

Edit: This is somewhat speculation on my behalf, I don’t have any more information that you, but I guess we will find out for sure soon enough.