OS X 10.5 is shipping today under the Leopard moniker. Besides being a great upgrade to a wonderful operating system, it's also the first version of OS X that ships with Rails in the package. Apple has done a phenomenal job including all the good stuff from the Ruby and Rails world into the developer tools that come with the OS.
So out of the box you get Ruby 1.8.6, Rails 1.2.3 (which is just a "gem update rails" call away from being 1.2.5), Capistrano, SQLite-bindings, and so much more. No more need for compiling your own Ruby. It's great. See all the changes in What's New in Leopard.
The only minor snag is that in order to install the MySQL C bindings for Ruby, you have to be quite particular on the command line. Here's the cheat line you need to install (read more at macosforge):
sudo env ARCHFLAGS="-arch i386" gem install mysql -- --with-mysql-config=/usr/local/mysql/bin/mysql_config
So happy Leopard day, folks!