In experimenting with Rails on Mac OS X, I’ve found that Locomotive is the easiest way to get set up and moving. It includes Ruby and Rails, in recent versions, and installs in an entirely self-contained way that won’t clash with any existing installations of Ruby, Rails, gems, or anything else you have on your system. It also includes a Ruby that’s been compiled with a few goodies (like GNU readline) that the version distributed with Tiger lacks.