Duke Nukem Forever seems to use just predefined textures as ladders, no special actors or zones like on Unreal or its mods..
It takes this rather from the Build side of things. There are positives and negatives to this method though.
Now here I want to talk about what to do if you want something to be climbed but don't wanna use one of those textures.
Just place an invisible mover that has the climbable texture... this is the cleanest method. All surfaces need to have the invisible flag but the engine won't care since the player is touching the ladder surface.
Easy solution really.
I think it can be used the other way too, to have a mover in front of a visible ladder texture not to make it work or have a copy of the texture...