The Apple Developer Connection has a nice Ruby on Rails tutorial called Using Ruby on Rails for Web Development on Mac OS X. As the title suggests, it is aimed at getting OS X users off the ground with Rails.
The Apple Developer Connection doesnt supply any attribution for their articles, but this one was written by none other than Mike Clark, who along with Dave Thomas runs the always-sold-out Pragmatic Studio series of Ruby on Rails and Ajax training.
Cheers to Mike for taking the time out of his busy schedule to write up a great tutorial. [Riding Rails]