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/<
Using ruby 1.9.3, I was able to get this to work by adding the gem 'ruby-debug-base19x' (note the 'x' at the end of the name) in addition to 'ruby-debug19'.