In the last 6 months I have installed Ruby on Rails 3 a hand full of time on OS X.  There are lots of different articles on how to do this but this is the simplest and the best way I have found to setup my Ruby on Rails 3 development environment.

Install XCode

• You need to install Apple's development tools / XCode

Installation

Install Hombrew

• https://github.com/mxcl/homebrew

Install Git (with homebrew)

• brew install git

Install RVM (Ruby version manager)

• http://beginrescueend.com/rvm/install/

Install Ruby 1.9.2 (or whatever version)

• rvm install 1.9.2

Install Rails 3

• Rails is a gem, so you can include this in your new Rails projects gem file and have it available. In your .Gemfile it is just
•   gem 'rails', '3.0.9'