You'll want to change the URL it's using, but the below is the correct way to do it.
require 'rubygems'
require 'RocketAMF'
require 'rest-client'
data = [] # whatever data you want
env = RocketAMF::Envelope.new :amf_version => 3
env.messages << RocketAMF::Message.new('BatchController.authenticate_iphone', '/1', data)
res = RestClient.post "http://localhost:9292/amf", env.to_s, :content_type => 'application/x-amf'
puts RocketAMF::Envelope.new.populate_from_stream(res).inspect