Good news! I've got Ruby 1.9.3p194 installed on Mountain Lion, here's how I did it:

1. Install Command Line Tools from Xcode by going into Xcode | Preferences | Downloads and installing straight from there.
2. In a terminal window run export ARCHFLAGS="-arch x86_64"
3. I had to "unlink" my copy of deadline by running "brew unlink deadline"
4. Install the latest version of deadline by running "brew install readline"
5. Install libxml2 via "brew install libxml2"
6. 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

Good luck!