> Veterans can provide more succinct ways though :) 

I wouldn't consider myself a veteran yet, but here's how Rails does it:

camel_cased_word.to_s.gsub(/::/, '/').
      gsub(/([A-Z]+)([A-Z][a-z])/,'\1_\2').
      gsub(/([a-z\d])([A-Z])/,'\1_\2').
      tr("-", "_").
      downcase

That also changes :: to /, so it's handy for translating a module name 
to a file path.  It's not exactly more succinct, but you can cut it down 
as you see fit. 

Brett