If you've tried to install ruby-debug with ruby 1.9.x there are two things you need to know. First, you need to be using the
ruby-debug19 gem and not the regular
ruby-debug one. Second, you'll probably need to point to the ruby source for the version of 1.9 that you are using. And that is as simple as:
gem install ruby-debug19 -- --with-ruby-include=`cd ~/.rvm/src/ruby-1.9.2-preview3; pwd`
1.9.2-preview3 above with whatever release you are using.