In reply to Thatoneguy:
Personally, I look upon any sort of helmet as a tool that clever people have spent time designing for a specific sport and to do a particular job (i.e. defend against a particular magnitude of impact, potentially on a particular surface, to disperse several small impacts vs. one biggie etc.).
The style, location and type of impact cycling helmets are designed for (I'm no expert, but I imagine one major hit, with potential high-risk zones on the front, back and sides) is different to what climbing helmets need to withstand (lots of light-ish bashing and tapping and maybe the odd heavy thing dropped on the top of it???).
I like my head, so I buy the right tool for the job