This fix will enable Control+R and UTF-8, which by default don’t work on Snow Leopard because OSX’s Ruby doesn’t use readline by default. All code comes from Henrik Nyh’s blog, which explains how to fix irb on Leopard. I’ve changed the code so that it works on Snow Leopard:

   1  sudo port install readline +universal
   2  svn co http://svn.ruby-lang.org/repos/ruby/tags/v1_8_7_72/ext/readline/ readline
   3  
   4  curl http://pastie.textmate.org/pastes/168767/download | patch readline/extconf.rb
   5  
   6  cd readline
   7  ruby extconf.rb
   8  make
   9  sudo make install