Guys, you will be able to play online co-op on the PC and PS3 using only the PS3 version with the free PC download code. With the new FAQ and reading dev replies over at the Steam forums, this is indeed possible.
1. Boot up Portal 2 on your PS3 and create/link your main PSN account to Steam from inside the game.
2. Log-off your main PSN account and log into one of your alternate PSN accounts.
3. Boot up Portal 2, and link this account to a new Steam account (Obviously different from the one you linked to with your main PSN account). At this point you will now have two completely separate PSN/Steam accounts on one PS3.
4. Enter the PC download code provided with the PS3 version of Portal 2 for this PSN account, and then do whatever is required to download the game on Steamplay.
5. Log-off this PSN account and go back to your main account, boot up Portal 2, log into Steam and go online to co-op.
6. Goto your PC and boot Portal 2 using the different steam account you created with your alternate PSN.
7. Add each-other as friends and enjoy online co-op over Steam using only one version of the game (PS3 version of course).
Here is why I believe this will work. From the FAQ it is clear that the PS3 version of Portal 2 has a code inside the gamecase (separate from the disk) that will allow you to download the PC version on Steam. From what I have read, this code is not required to create or link a PSN/Steam account. It is only needed when you want to download the PC version on Steam.
With that said, VALVe stated that multiple PSN and Steam accounts can be played from one PS3, meaning this... you can use that PC code for ANY PSN account you want and is not restricted to just your main account, as it is not required to play or link a PSN account to a Steam account.
Some facts to backup my theory. Proof that the code is separate from the gamedisk.
From the FAQ posted here - https://support.steampowered.com/kb_article.php?ref=5477-WPJM-9388
When you purchase a new copy of Portal 2 for PlayStation®3, you will receive an in-box code which provides your linked Steam account with access to your copy of Portal 2 on your computer.
Below is proof of concept that you can have multiple PSN/Steam accounts on one PS3. This also proves you do NOT need the PC download code to register/link Steam to a PSN account. Meaning you can use the PC download code you got with your PS3 version of Portal 2 on ANY
PSN account you want (only one time of course).
Online co-op on PS3 requires a Steam login, but you can auto-create a Steam account when first playing co-op with a single button. You don't have to type in a username or password, you don't have to do anything. It's super easy, you can also do this multiple times for different PS3 local user profiles/PSN accounts, so for instance you and your brother can both have separate PSN and Steam accounts and play the game online on PS3, just not at the same time with a single disc of course. It's not meant to be intrusive DRM in anyway, in fact Steam on PS3 is not DRM at all, it's there to improve the multiplayer matchmaking experience and to support cross platform play.
For split-screen you do not need to link/create a Steam account and you can play totally offline.