Install Ruby support for VIM on Mac OS X
While it's possible to build and install your own Vim to replace the pre-installed version, I don't recommend it. It's far easier to just use MacVim instead:
http://code.google.com/p/macvim/
MacVim is a very Mac-friendly version of Vim, and it's got Ruby support already built in. It can be used as both a GUI and Terminal application. (Check out :help macvim-start from within MacVim for details.)
+1 for MacVim - I also use it and love it.
But if you want a fresh vim compiled with ruby support, you can build one from scratch on OSX by following this article:
http://brilliantcorners.org/2011/02/building-vim-on-osx-snow-leopard/
Only took me about 5 minutes to do...