The Ruby on Rails scene for new users on the Mac has and continues to be pretty confusing! The version of Ruby that comes with the Mac isn’t suitable so you have to install a second copy and if you go to the getting started page at rubyonrails.org the first thing you see for OSX recommends building readline, ruby, fastcgi, lighty, and pcre from source. Fortunately Geoffrey at TopFunky has released a script that does all of that for you which looks like a definite improvement (although I haven’t tried it on my MacBook yet).
The only really viable binary option for the Mac appears to be Locomotive which is an all-in-one package (quite nicely done) but hasn’t been released as a universal binary yet and is based on Ruby 1.8.2.
Having switched from Windows recently I can say that getting started with the latest versions of Ruby on Rails on Windows was more straight forward: download Ruby binary, download Gems, run gem install rails, and off you go. In the meantime I’m going to stick with Locomotive which works fine with Rosetta emulation in the hopes that a new universal binary with Ruby 1.8.4 is on the near horizon!
Perhaps if I get some time I’ll take Geoffrey’s script and change it to install everything in an alternate directory such as /usr/local/ror and then tar it up that you can just extract it to /usr/local/ror, set your environment and go. Then when you want to upgrade later you can just rm -rf /usr/local/ror without worrying about which files under /usr/local you can or can’t delete.