I'm pretty interested in that project. We had a dev on Lion try to get a Ruby 1.9.3 + Rails 3.2 environment up who ran into all kinds of issues: a far cry from the days of locomotive indeed.
for future reference, step 1 - gcc toolchain (https://github.com/kennethreitz/osx-gcc-installer or xcode), step 2 - rvm (http://beginrescueend.com/), step 3 - "rvm install 1.9.3 && rvm 1.9.3 --default && gem install rails" (homebrew will also probably be necessary for any dev work on osx, but not really needed to get rails going)