Install Apple Developer Tools (xcode + cli)
Install Homebrew:
http://mxcl.github.com/homebrew/
Once homebrew is installed you need install gcc42:
# brew tap homebrew/dupes
# brew install apple-gcc42
Add this line to your ~/.profile:
export CC=/usr/local/bin/gcc-4.2
Then run:
# sudo ln -s /usr/local/bin/gcc-4.2 /usr/bin/gcc-4.2
Then reinstall RVM:
https://rvm.io/
Then you can install what ever version of ruby you need:
# rvm install 1.9.3
You can run rvm requirements
and it will search for missing dependencies.