À la VCR.

open-uri-cache.rb

require 'open-uri'
require 'digest/md5'

class << OpenURI
  CACHE_DIR = Rails.root.join 'tmp', 'cache', 'open-uri'
  CACHE_DIR.mkdir unless CACHE_DIR.exist?

  alias cache_original_open_uri open_uri
  def open_uri name, *rest, &block
    snap = Digest::MD5.hexdigest name.to_s
    path = CACHE_DIR.join snap
    return path.read if path.exist?
    io = cache_original_open_uri name, *rest, &block
    path.open('wb') { |f| f.write io.read }
    io.rewind
    io
  end
end