My coworker Krishna, inspired by Giles Bowkett's post Rails Debugger: Use RCov With Selenium, Watir, Or Just In Real Life, whipped up a little rake task that automates the process of checking code coverage using your Selenium tests:

desc "Measure coverage of selenium specs"
task :coverage do
  FileUtils.rm_rf "selenium_coverage"
  mongrel_pid = fork { exec("rcov script/server -o selenium_coverage --rails -- -e test -p 4000") }
  sleep(6) # wait for mongrel to start

  begin # selenium will call "exit 1" if there are failures
  rescue SystemExit; end

  Process.kill("INT", mongrel_pid) # triggers generation of rcov report
  system("open selenium_coverage/index.html") if PLATFORM['darwin']

While we don't agree that this task, in any way, shape, or form, constitutes a debugger, it has been useful in showing us areas of our code base that our Selenium test cases aren't hitting.

This should work for just about anyone that has a rake task to run their Selenium (or Watir, or other in-browser testing tool) tests. Just replace 'selenium:spec' in the block above with whatever task you use to run your in browser functional tests and have at it.