This functionality is built into Ruby on Rails. I don't know exactly where, but it should be easy enough to find in the source code, and then you could simply crib the code.
EDIT: Found you some code:
- inflector.rb (very helpful comments!)
- inflections.rb (extensive word list)
If I remember correctly, it's mainly a matter of appending an "s" to most words, though I believe there is a list (probably hash, err dictionary) of some common exceptions. Notable is the conversion from "person" to "people" :)
You would of course be in for a world of pain if you decided you want to internationalize this to other languages than English. Welcome to the world of highly irregular grammars, and good luck!
