I spent a good six months of my life searching with about 20 others for secrets in shadow of the colossus. You wouldn't believe the theories and research and exploring we did. We had a hard, hard time believing that the Shrine Of Worship was only climbable to a certain point. There had to be a way to the top. Sadly there wasn't. One of the guys started emulating the game and ended up glitching himself up there, and he also found a large dam/bridge structure outside of the map that was some scrapped idea left in by the developers. Here's the thread where we wasted all of our time:

http://community.us.playstation.com/thread/2267051?start=0&tstart=0

Here's the guy glitching himself to the top and wandering around. He had to manipulate the games code to make the textures on the shrine solid.




Here's the dam that is in the in-accessible eastern area of the map:



And here's your's truly using a rudimentary method (hawk-humping) to try and get to a ledge on the side of the shrine: