The closest experience I've had to this is using RSpec and Capybara to do test-driven development.

Here's an actual snippet of a test I wrote before writing the corresponding code for an app that's now in production:

  describe "adding participants" do                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
    context "when user has created a new event" do
      before :each do
        visit root_path
        click_link 'Get Started'
        fill_in 'Name', :with => 'Tres'
        fill_in 'E-mail', :with => 'tres@sogetthis.com'
        click_button 'Next'
      end
    
      it "should prompt for more participant info" do
        page.should have_field 'Name'
        page.should have_field 'E-mail'
        page.should have_button 'Add Participant'
      end
    
      it "shouldn't let you enter an invalid e-mail" do
        fill_in 'Name', :with => 'Tres'
        fill_in 'E-mail', :with => '#)(*)($*#)($*'
        click_button 'Add Participant'
        page.should have_content 'invalid'
      end
    end
  end

It's been an absolute joy to work this way. I'd love to see this level of abstraction make its way into the mainstream in other languages and environments.