Made an update covering Macos 10.8 and ruby 2.0: https://gist.github.com/michaelsd/5224456

- Install "Command Line Tools for XCode" (Get it from Apple Developer Program) or Install XCode via Appstore
where option one is the faster one, 170MB vs 1.xGB

- Install https://github.com/sstephenson/rbenv
git clone into your home folder;
set environment variables, see install docs of rbenv;

- Get libyaml
wget http://pyyaml.org/download/libyaml/yaml-0.1.4.tar.gz
configure
make && make install
Remember installation path: /usr/local/lib will be used to configure Ruby Source Code

- Get Ruby 1.9.3 Source
wget http://ftp.ruby-lang.org/pub/ruby/1.9/ruby-1.9.3-p125.tar.bz2

- unpack Ruby with tar -xzf ruby-1.9.3-p125.tar.bz2

- Configure, compile and install Ruby: (This is just one way to do it. On a clean new MacOS with XCode 4.3 there is no GCC Compiler anymore, so we have to use clang. This method was not supported by the ruby-build utility by the time of writing this)
./configure --prefix=$HOME/.rbenv/versions/1.9.3-p125 --with-gcc=clang --with-opt-dir=/usr/local/
make
make install

- Issue the following commands:
rbenv rehash
rbenv global 1.9.3-p125

Ruby should now be installed, test with ruby --version which should return something like:
ruby 1.9.3p125 (2012-02-16 revision 34643) [x86_64-darwin11.2.

- Install gem:
wget http://rubyforge.org/frs/download.php/75856/rubygems-1.8.17.tgz
Unpack and ruby setup.rb in folder

- If we get a file 'lib' not found error:
gem install rdoc

- Install rails
gem install rails

- If you run the rails command and it could not be found, go to /usr/bin/
and do a sudo mv /usr/bin/rails /usr/bin/rails.old and restart your shell

- Another Problem solved with missing XCode dependencies:
gem install rb-fsevent failed with Error extconf.rb:15:in `<main>': Could not find a suitable Xcode installation (RuntimeError)
Solutions: install rb-fsevent >= 0.9 which comes with an prebuild binary should help
references: https://github.com/thibaudgg/rb-fsevent/issues/31
and https://github.com/thibaudgg/rb-fsevent/issues/20