Question: And where can I find it? I'm running OSX.

In addition I have to do a lot of work offline while travelling so if anyone can offer any tips for downloadable documentation please let me know.

Answer: 
If you have installed your gems the "normal" way, they install rdoc documentation.

Just run `gem server` in your console and go to <http://localhost:8808> in your browser. There you'll find all available Gems and the RDoc documentation
<http://railsapi.com> provides you to download offline documentation for:

* Rails 2.2.2, 2.3.8, 3.0.8
* Ruby 1.8, 1.9.2
* Authlogic
* AWS-S3
* EventMachine
* Haml
* Hpricot
* Nokogiri
* Rack
* Rspec
* Sinatra

The downloadable file has wonderful jQuery search functionality, so it's pretty useful