Will generate a heroku gem manifest from the rails app itself. Ideal as a sake task.

heroku_gem_manifest.rake

desc "Generate Heroku gems manifest from environment.rb"
task 'heroku:gems:manifest' do
  manifest = nil
  IO.popen('-') do |io|
    if io
      manifest = io.read
    else
      system 'script/runner', %q{
        out = Rails.configuration.gems.collect do |gem|
          gem.send(:install_command)[1..-1] * " "
        end * "\n"
        puts out
      }
    end
  end
  File.open('.gems', 'w') {|f| f << manifest}
end