It also looks like Walter brought his son back from the alt-reality after he died at the age of 7.
That to me was the huge shocker. I was wondering whose grave Walter was visiting, and it explains why Peter's childhood memories don't mesh with Walter's -- it also begs a lot of new questions. Is Peter's mother still alive in the alternate reality? Are she and Walter still together? What will Peter's reaction be when he finds out?
While the Twin Towers were a shock to see, when they panned onto that gravestone with the "Peter Bishop, 1977-1985" I truly got chills up my spine.
Fringe is one of the best written shows in years in my opinion. Full of interesting characters, and a story line/underlying plot that just gets deeper and deeper with every episode. J.J. Abrams is truly a master story teller.