I would recommend MacPorts approach as well. Gives you system independence (apple updating ruby, etc) and you can easily replicate your development environment on multiple machines regardless of the OS version. Duncan's guide isn't as complex as it seems, it just includes some steps for it to include some user friendliness in the long run. Steps 1 through 3 is just installation of MacPorts, Steps 4, 5 are Ruby and Rails, but you don't really need to install all of them. For simple setup, just install ruby+rails+mongrel+whatver gems you need, though I seriously recommend subversion+capistrano, if you are even a little bit serious about the development.

For the simplest setup, you could just go with the Ruby that's included in OSX, install gem support and install everything from gems, but be aware that there are some version and other issues with Rails and the Ruby Apple ships bundled in the OS.

In any case, welcome back to the Rails works and have fun :) Feel free to drop me a line if you need anything.