I know he's not in the red universe (Walternate's universe) since the twin towers weren't destroyed there. Someone on Fringepedia suggested that he might have created an new alternate universe, but I kinda doubt that. I think it's the blue universe (ours).
As for Olivia being the "crowbar," I totally called that the moment I saw the rolled up piece of paper. However, when Walter said she should practice on something smaller before trying to open the force field around the machine (which is apparently called the Wave Sink Device), and they
didn't pull out the tests that David Robert Jones gave her in season 1, I was very upset.
I have
no idea how Peter going into the future is supposed to save anything, but it did seem in the preview that the red universe gets destroyed sometime between now and 10 years from now. At least, it sounded like Walternate said, "You destroyed my world. Now I will destroy yours!" or something along those lines. Then again, he may just be refering to Walter crossing over and starting all the rifts and whatnot back in 1985.
Oh, and before I forget, I hope there's some significance to Peter losing his memory for a while and getting that silver half dollar. It didn't seem to have any purpose except to scare Walter when he disappeared from the hospital.