If you're using homebrew and rbenv on Mac, and want to get on the Ruby 2.0 bus, do this
brew upgrade rbenv brew upgrade ruby-build rbenv install 2.0.0-p0
If all goes well, you should see something like the following -
rbenv versions ... 2.0.0-p0 ...
Start rocking with your new shiny Ruby 2.0 !
rbenv local 2.0.0-p0 gem install bundler
And if you've itching to get on the Rails 4.0 bus as well, here you go -
gem install rails --version 4.0.0.beta1 --no-ri --no-rdoc