Currently Rubygems.org is down - the website reports an error, and a few gem install tasks are returning 500 errors.
Is there a mirror / backup source of gem files, or is rubygems.org essentially a single point of failure within the ruby gem installation process ?
As per several folks on Twitter, add this to your Gemfile:
Also, if you're not using Bundler then just run:
$ gem source -a 'http://production.cf.rubygems.org'
For the record, the folks on Twitter include: