Having just installed Mountain Lion on my MBA, I encounted a few small problems while trying to install RVM as well as Ruby, etc. Below are the steps I took to get Ruby on Rails up and running on my new Mountain Lion install.
Step 1. Install XCode (this isn't required but it's best to install it)
Step 2. Install the XCode Command Line Tools from developer.apple.com. Note you need to download the 10.8 command line tools which is at the bottom of the list on XCode 4.4. I made the mistake of downloading the 10.7 Lion set first!
Step 3. Install XQuartz
Step 4. Install HomeBrew
ruby -e "$(curl -fsSkL raw.github.com/mxcl/homebrew/go)"
Step 5. Check that HomeBrew works OK
Step 6. Install GCC 4.2 because Mountain Lion doesn't come with it by default.
brew tap homebrew/dupes brew install apple-gcc42 sudo ln -s /usr/local/bin/gcc-4.2 /usr/bin/gcc-4.2
Step 7. Install RVM
Step 8. Install Ruby!
rvm install 1.9.3 for the latest version of Ruby 1.9.3
Step 9. Set 1.9.3 as the default ruby -
rvm use 1.9.3 --default
Step 10. Install the base gems -
gem install rails bundler
Step 11. Install Pow which is a brilliant Ruby development server. No need to keep running
rails server all the time.
curl get.pow.cx | sh
Step 12. Install Git
brew install git
Step 13. (optional). Install MySQL using homebrew
brew install mysql. It looks like Java is required as well during installation for some reason.