Enter the following script as user data while firing up new instances and you will find a basic ruby environment, e.g. in order to start your chef provisioning.

Amazon Linux AMI Bootstrapping

#!/bin/bash

yum -y install make gcc-c++ zlib-devel openssl-devel
yum -y install ruby-devel ruby-irb ruby-rdoc ruby-ri

cd /tmp
curl -O http://production.cf.rubygems.org/rubygems/rubygems-1.3.7.tgz
tar xfz rubygems-1.3.7.tgz
ruby rubygems-1.3.7/setup.rb

gem install chef ohai --no-rdoc --no-ri

mkdir /etc/chef
chown ec2-user:ec2-user /etc/chef

ssh config

Host *.compute.amazonaws.com
  User ec2-user
  IdentityFile ~/.ssh/testing.pem

Ubuntu 10.04 LTS (Lucid Lynx) AMI Bootstrapping

#!/bin/bash
# Find AMIs here:
# http://uec-images.ubuntu.com/releases/lucid/release/

apt-get update
apt-get -y install ruby ruby-dev libopenssl-ruby rdoc ri irb build-essential wget ssl-cert rubygems1.8

gem install chef ohai --no-rdoc --no-ri

mkdir /etc/chef
chown ubuntu:ubuntu /etc/chef