First, make sure you ran the XCode installer that was downloaded from the AppStore. Then run the Mac OS X 10.7 gcc installer
Check that gcc-4.2 is listed
$ ls /usr/bin | grep gcc
gcc
gcc-4.2
i686-apple-darwin11-gcc-4.2.1
i686-apple-darwin11-llvm-gcc-4.2
llvm-gcc
llvm-gcc-4.2
Install Ruby 1.9.3 Success!!
$ rvm install 1.9.3
Fetching yaml-0.1.4.tar.gz to /Users/mdesilva/.rvm/archives
Extracting yaml-0.1.4.tar.gz to /Users/mdesilva/.rvm/src
Configuring yaml in /Users/mdesilva/.rvm/src/yaml-0.1.4.
Compiling yaml in /Users/mdesilva/.rvm/src/yaml-0.1.4.
Installing yaml to /Users/mdesilva/.rvm/usr
Installing Ruby from source to: /Users/mdesilva/.rvm/rubies/ruby-1.9.3-p0, this may take a while depending on your cpu(s)...
ruby-1.9.3-p0 - #fetching
ruby-1.9.3-p0 - #extracted to /Users/mdesilva/.rvm/src/ruby-1.9.3-p0 (already extracted)
ruby-1.9.3-p0 - #configuring
ruby-1.9.3-p0 - #compiling
ruby-1.9.3-p0 - #installing
Removing old Rubygems files...
Installing rubygems-1.8.10 for ruby-1.9.3-p0 ...
Installation of rubygems completed successfully.
ruby-1.9.3-p0 - adjusting #shebangs for (gem irb erb ri rdoc testrb rake).
ruby-1.9.3-p0 - #importing default gemsets (/Users/mdesilva/.rvm/gemsets/)
Install of ruby-1.9.3-p0 - #complete