Good news! I've got Ruby 1.9.3p194 installed on Mountain Lion, here's how I did it:
- Install Command Line Tools from Xcode by going into Xcode | Preferences | Downloads and installing straight from there.
- In a terminal window run export ARCHFLAGS="-arch x86_64"
- I had to "unlink" my copy of deadline by running "brew unlink deadline"
- Install the latest version of deadline by running "brew install readline"
- Install libxml2 via "brew install libxml2"
- and finally, rvm install 1.9.3 via "rvm install 1.9.3 -C --with-readline-dir=/usr/local/Cellar/readline/6.2.4/ --with-libxml2-dir=/usr/local/Cellar/libxml2/2.7.8/" - pay special attention to the path and do use tab completion in the terminal window to help prevent any typo's