I set the Options->Miscelaneous->Ruby configuration options to use Ruby 1.8 (/usr/bin/ruby for me).

Next I created a simple Ruby on Rails project using NetBeans 6.0M10.  Call the program "Demo"

I created a controller (test) and a view (index) and deliberately introduced a run-time error in my controller code (two
render's within one method.

class TestController < ApplicationController
   def index
      render :text => "Line 1"
      render :text => "Line 2"
   end
end

Now I run the application using the Run item on the Project's pull down menu.

Navigate to the location http://localhost:3000/test/index

The error is correctly identifed, but the ruby interpreter is jruby instead of the ruby setting from my options.