Very nice plugin, but on Windows you see on the console
... ps : illegal option -- o Loaded suite C:/ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake/rake_test_loader Started ... That 'ps' line is due to init.rb, line 3

Assumes Unix (Linux?); doesn't work on Windows. Simple fix, don't do this on Windows -- wrap the current while...end loop with a not-windows check:

if ENV['RAILS_ENV'] == 'test' && !%w{false none}.include?(ENV['BENCHMARK'])

unless RAILS =~ /mswin32/
  while (pid ||= $$).to_i > 0
    pid, *process = `ps -p #{pid} -o ppid -o args`.strip.split("\n").last.split
    @autotest ||= process.join =~ /autotest/i
  end
end

require File.dirname(__FILE__) + "/lib/test_benchmark" unless @autotest

end