I had a rails 3.0 app where I wanted to do some jQuery. So first step, convert from prototype/scriptaculous to jquery. So head over to https://github.com/rails/jquery-rails and follow the instructions there, essentially add to your Gemfile:
Run bundler to install the gem, then run the generator to update the app to use jquery:
bundle install rails generate jquery:install
After this everything worked fine,
DEPRECATION WARNING: config.action_view.debug_rjs will be removed in 3.1, from 3.1 onwards you will need to install prototype-rails to continue to use RJS templates . (called from service at /Users/jeffw/.rvm/rubies/ruby-1.9.2-p180/lib/ruby/1.9.1/webrick/httpserver.rb:111)
But this was a simple case of removing the line:
config.action_view.debug_rjs = true
config/environments/development.rb as discussed at http://weblog.rubyonrails.org/2011/4/21/jquery-new-default.