Hi, I'm having what i believe to be the same issue.
I have Modloader. Hope thats not the problem. Inventory Tweaks
Forge. Rei Minimap and Optifine HD B3
I noticed the ladders immediately, but out of curiosity, jumped into creative, and have the same problem with Trapdoors and Vines (Cant Climb)
I have them installed via Magic Launcher, with all Internal except Inventory Tweaks.
Hope thats enough information, hope you can help.