I apologize in advance if these questions have already been answered and the fact that there are actually three, but my copy of Portal 2 just came in today and I've been on something of an information blackout with this game. I'm also not entirely certain if two of these questions can be answered because they pertain to the PS3 version's Steam support.
1) Is syncing your PSN and Steam account necessary for cross-platform play?
2) Do Steam achievements translate into Trophies retroactively?
3) How connected is the co-op campaign to the single-player, both in terms of narrative and gameplay (ie; do you have to beat the single-player to get access to the co-op challenges)?