By default ruby is installed with libedit library on OSX. This is really annoying especially with IRB, as default bash bindings (ie ^R for reverse history search) do not work. There’s workaround to enable history search, but now it’s event easier just to install ruby with readline enabled (RVM installation assumed), just run:
bash < <(curl http://github.com/gmarik/dotfiles/raw/master/bin/osx/rvm-install-ruby-readline)
see source for more details.