I had to do a bit of running around to get Rspec, Cucumber, and rcov working together nicely; here’s my current rake task:

require 'cucumber/rake/task'

require 'spec/rake/spectask'

namespace :rcov do

  Cucumber::Rake::Task.new(:cucumber) do |t|    

    t.rcov = true

    t.rcov_opts = %w{--rails --exclude osx\/objc,gems\/,spec\/,features\/ --aggregate coverage.data}

    t.rcov_opts << %[-o "coverage"]

  end

  Spec::Rake::SpecTask.new(:rspec) do |t|

    t.spec_opts = ['--options', "\"#{RAILS_ROOT}/spec/spec.opts\""]

    t.spec_files = FileList['spec/**/*_spec.rb']

    t.rcov = true

    t.rcov_opts = lambda do

      IO.readlines("#{RAILS_ROOT}/spec/rcov.opts").map {|l| l.chomp.split " "}.flatten

    end

  end

  desc "Run both specs and features to generate aggregated coverage"

  task :all do |t|

    rm "coverage.data" if File.exist?("coverage.data")

    Rake::Task['rcov:rspec'].invoke

    Rake::Task["rcov:cucumber"].invoke

  end

end

Running rake rcov:all should give you nice coverage data that includes both rspec and cucumber.