If you have ever tried to update rubygems on a base Ubuntu 10.04 or 10.10 install you will have been greeted with:
# gem update --system
ERROR: While executing gem ... (RuntimeError)
gem update --system is disabled on Debian. RubyGems can be updated using the official Debian repositories by aptitude or apt-get.
You can work around this limitation:
# gem install rubygems-update
You will now be able to update rubygems using the standard set of commands.