Add the following code into a file called lib/tasks/rcov.rake. Notice that I used the find to gather the relevant tests then the xargs to form them into a nice line which can be added to the rcov command.

namespace :test do

  desc 'Tracks test coverage with rcov'
  task :coverage => :environment do
    rm_f "coverage"
    rm_f ""

    rcov = "rcov --sort coverage --rails --aggregate --text-summary -Ilib -Itest -T -x gem/*,rcov*"

    files = `find . -name *_test.rb | xargs`

    rcov = "#{rcov} " + files
    puts rcov
    system rcov

    system("firefox coverage/index.html")

The new rake command can be executed using rake test:coverage