rvm-your-chef.rb

# Check this out, rvm use rbx

ruby_block "rvm use rbx" do
  block do
    Chef::Mixin::Command.popen4('bash -l -c "rvm use 1.9.1 && env"') do |p,i,o,e|
      o.each_line do |line|
        env_bits = line.split("=")
        ENV[env_bits[0]] = env_bits[1]
      end
    end
  end
end

# All future stuff should get the rvm environment

gem_package "rails"

You need a chomp on line 7 to prevent newlines from getting into your env vars. See my fork (https://gist.github.com/928628).