I thought I'd write up the steps I took to get Drupal running on a stock Leopard installation. You may wish to save some time and install MAMP instead. Especially if you need GD support (i.e., you're going to have Drupal do image resizing). Update 17-Dec-2007: In fact, I recommend using MAMP instead.
Step 1: Enable PHP
Uncomment line 114 in /etc/apache2/httpd.conf to enable Leopard's built-in PHP:
LoadModule php5_module libexec/apache2/libphp5.so Read more... about Installing Drupal on Mac OS X 10.5 Leopard