I have been working on updating my gem (whm_xml at https://github.com/ivanoats/whm_xml_api_ruby ) to make it work with ruby 1.9.2, latest rubygems, latest bundler, latest rd
This SO Question might help you out. The suggestion there is to add require 'rake/dsl_definition' above require 'rake' in your Rakefile.
require 'rake/dsl_definition'
require 'rake'
Rakefile