I have standard ruby-head and Rails 3.1rc4.
I installed ruby-debug with following instructions from http://dirk.net/2010/04/17/ruby-debug-with-ruby-19x-and-rails-3-on-rvm/<
If you are running Ruby 1.9.3x, to fix it go to ruby-debug on rubyforge, download and install ruby-debug-base 0.11.26 and linecache19 0.5.13. Modify your Gemfile to use these files, then run