I was planning to play co-op cross-platform with my brother, but PSN being down kinda got in the way of that. So could I play split-screen on my PSN account, beat co-op with him, and get my Steam achievements once PSN is back up? Not entirely sure there's a way to test this right now, but maybe someone here has come across the answer.

I started and finished the single player while PSN was down, and was under the assumption that given you get trophies, once PSN is back, and you sign into Steam, it just syncs up to your trophies.

Hopefully your right.  I never even thought about that. 

I'm fairly certain that the Steam achievements unlock if the trophies are unlocked, so once you sync it up you should be good to go. It doesn't work the other way around though (so Steam achievements don't unlock PS3 trophies.)

Well, Steam achievements straight up don't unlock if you get them while you aren't connected to the Steam servers on PC.

Who knows how they handled the PS3-Steam sync, though.

There's a Portal achievement that I unlocked in the game but never unlocked on Steam because I was somehow offline at the time. I wouldn't risk it after that.

On the topic of PSN problems, I'm slowly but surely getting somewhat pissed about not being able to activate my Portal 2 steam company. I know Valve is not really responsible for the entire mess, but it would be nice to see a workaround of some sorts. Really dying for that multiplayer content after finishing the single player campaign a couple of times, including the dev commentary....

" On the topic of PSN problems, I'm slowly but surely getting somewhat pissed about not being able to activate my Portal 2 steam company. I know Valve is not really responsible for the entire mess, but it would be nice to see a workaround of some sorts. Really dying for that multiplayer content after finishing the single player campaign a couple of times, including the dev commentary.... "