To my dismay, Ruby on Rails seems to be linked deeply into OS X's frameworks, which I don't want to mess with. Some poking reveals that the default installation, version 1.8, is in a system framework called Ruby.framework. The normal method to updating such tools is to remove the old version and then install the new one, but I did not want to take the risks this time. Turns out there's a great tool called rvm that will manage multiple Ruby versions!
Complete instructions are here: http://bigdiver.wordpress.com/2011/11/24/update-ruby-and-rails-on-mac-os-x/
They're written with OS X in mind, but adapting them for any Linux system should be trivial.