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rcov 0.7.0 can aggregate code coverage and defsite/callsite data collected in successive runs. This is useful when you want to obtain a global coverage rate but your tests are split into several parts, for instance units, functionals and integration, as happens in Rails.

Here's how you'd define coverage tasks for unit, functional and integration tests under the test:coverage namespace, as well as a test:coverage task that runs them all and yields an aggregate coverage rate:

require 'rcov/rcovtask'

namespace :test do 
  namespace :coverage do
    desc "Delete aggregate coverage data."
    task(:clean) { rm_f "coverage.data" }
  end
  desc 'Aggregate code coverage for unit, functional and integration tests'
  task :coverage => "test:coverage:clean"
  %w[unit functional integration].each do |target|
    namespace :coverage do
      Rcov::RcovTask.new(target) do |t|
        t.libs << "test"
        t.test_files = FileList["test/#{target}/*_test.rb"]
        t.output_dir = "test/coverage/#{target}"
        t.verbose = true
        t.rcov_opts << '--rails --aggregate coverage.data'
      end
    end
    task :coverage => "test:coverage:#{target}"
  end
end

All you have to do to combine data from multiple runs is to add

 --aggregate FILE

so that the data from previous runs saved in FILE is loaded (if FILE doesn't exist then history will be empty), merged with the newer information and stored back into FILE: