"I ran into an issue today installing the ruby mysql gem on a fresh Leopard system with 10.5.1 installed and the MySQL 5 package for Intel installed. This is based on Dan Benjamin’s great work. To keep this easy, here is the low down. What was happening was an issue with it expecting headers for PPC. PPC? What the??? I’m on intel damn it!" - Mackenzie Cowell from Bookmarklet

Thanks for that link -- I ran into that exact problem today. You don't need the mysql gem (C implementation) right away though. For development purposes, Rails ships with a Ruby-based MySQL library. When you connect to the database the first time, it even says something like Hey, you'll want to install the mysql gem for production environments. - Paul Morganthall