Rails 3 documentation is finally out for Mac OS X Dictionary.app. If you're not sure what this is about, please have a look at my previous posts . Download your copy of Rails 3 Dictionary here.
Currently it is generated from Rails 3.0.0 beta 3 version. I will post the updates once the new Rails 3 versions roll out.
Unzip downloaded file into ~/Library/Dictionaries folder if you want to keep this dictionary only for yourself. You may need to create this folder if you haven't installed any dictionaries before.
To make it available for all users, drop the Ruby on Rails 3.0.0b3.dictionary folder into /Library/Dictionaries folder.