Install rvm on OSX 10.8.1 Mountain Lion in 4 easy steps.
  1. Download & install Command line tools for Xcode 4.4 (you don’t need to download Xcode):https://developer.apple.com/downloads/index.action
  2. Install Homebrew: https://github.com/mxcl/homebrew/wiki/installation
  3. Install automake: brew install automake
  4. Install RVM as usual and select 1.9.3-head as the default ruby installation: https://rvm.io/rvm/install/

Optional step: You may need XQuartz for some components, for example for Imagemagick, so download & install XQuartz: http://xquartz.macosforge.org/trac/wiki